Summary for overclocking CPU and FSB
When overclocking a system, there are two major components.
1. CPU clock frequency
2. FSB clock frequency which is related to memory bus clock freq and ras/cas timing
The two can be optimized separately and resolved by the CPU multiplier (to the first degree).
First new mb can lock PCI bus and AGP bus speed, so assume PCI is locked at 33 MHz, so don't have to worry about HD and video card AGP bus running out of spec.
1. CPU clock frequency: For a given CPU, it can be overclocked to its max frequency regardless of FSB speed, whether it is only running at 133 MHz or 166 MHz or oc to 220-250 MHz (for the newer mb). So even you have to keep an older mb (that can run 133 or 166 FSB) for a while, don't get discouraged and can still oc a powerful CPU such as the Tbred B 1700+ DLT3C or 2100+. The multiplier in the bios for AMD CPU will resolve that by
FSB x multiplier = CPU frequency
Assuming you have an unlocked CPU such as Tbred, e.g. if FSB = 200 MHz, you can set multiplier to 12.5 and so CPU, such as Tbred B 1700+ DLT3C may run at 2.5 GHz. Side track: some mb may cap the multiplier at 12.5 and you may need a mod to get to higher multiplier than 12.5. If needed you can find such info easily by a search.
CPU max oc frequency is mainly determined by
- the heat sink and fan (on CPU temp)
- Vcore and its stability (PSU rail and Vcore regulator cooling)
to the first degree, case and system temp secondary.
So for any system, assuming on air, if you have good heat sink such as SK7 (economical) or SLK-800U and variable speed fan with speed 2500-5000 rpm that can push air up to 75 CFM through the heat sink. The die temperature, such as Tbred B, should then be able to be kept under 50C at high level of oc. CPU fan speed can be lowered to between 3000-3500 rpm and the noise level is generally considered acceptable for 24/7 usage. AS3 or ceramique should help also.
Temperature is important not just because high temperature could damage the CPU (max temp from spec is actually much higher, 85-90 C for Tbred/Barton), more importantly lower temperature results in lower CPU leakage current which heats up the CPU drastically above certain temperature threshold (between 40-50 C), above which heat and current will positively feeding each other and the CPU will lose stability in oc'ing.
Good PSU delivers stable CPU Vcore (within +- 50 mV at full Prime95 load), given the high active current at max oc. For high oc to 2.5+ GHz at 1.9V Vcore may require good PSU to source enough active Icore current so that the Vcore rail, and the 5V or 12V PSU rail (depends on mb) that generates Vcore are stable. As an example, a CPU running at 2.5 GHz 1.9V consumes 2.7 times the active power, and draws 2.1 times the active current of that running at 1.5 GHz at 1.5V.
The Vcore regulator (those 4-6 big MOSFET's near the CPU socket are part of the regulator) on mb becomes important when overclocking to high clock frequency (2.5+ GHz) which requires large active current. Mb that derives Vcore from 12V PSU line has better current sourcing capability, I think. Sometimes max oc may be limited by the Vcore regulator stability and cooling, this is something that may be overlooked.
Even you have a mb running at slow FSB (133/166 MHz), you can still oc the CPU to highest possible frequency, using high mulitpliers (assuming multiplier limit is resolved). That is, max CPU oc freq is independent of FSB.
2. FSB clock frequency: Mb such as those w/ nforce2 chipset, PCI freq is locked, they can go at least to 180 MHz. If you are getting new mb currently, mb with nforce2 chipset is a good choice, new rev 2 mb with C1 chipset stepping supports least 200 MHz officially.
FSB and memory bus frequeny are locked at a ratio. If the ratio = 1 or 100% it is called SYNC mode, and is recommended for mb such as nforce2. If the ratio != 1, it is called ASYNC mode where FSB and memory bus are running at different frequency but in locked steps.
How high can FSB go depends on
- memory rated speed
- chipset quality (stepping) and chipset Vdd
- chipset (both NB and SB) cooling
To get higher FSB, make sure the memory module is rated to handle the target FSB at a given ras/cas timing and voltage. E.g. PC3200 memory will support to 200 MHz FSB (or DDR400), PC3500 for 217 MHz (DDR 434), ...
Memory timing should be set to 6-3-3-2 to begin with to get to as high a FSB as possible, then tighten it to x-3-2-2 or x-2-2-2 (where x = 5, 6), if possible but not necessary, after reaching max FSB. The difference between 6-3-3-2 and x-2-2-2 is minor (~2%) on overall system performance.
Dual channel or single channel mode in nforce2 mb is not that crucial for overall performance. The diference is only 2% at most. Also single channel may let FSB to go a bit higher due to minor potential dual dimm mismatch and memory controller stress at high FSB, I think. On the other hand, dual channel memory controller provides some performance advantage due to its intrinsic speculative caching capability. At this point, the little higher FSB from single channel tradeoff the performance advantage of dual channel, and the two is about a tie, I think, for AMD mb. Dual channel makes a big difference for P4 dual channel mb though, due to quad pump data of P4.
Chipset Vdd and Cooling on FSB stability
If wanting to go higher, chipset Vdd and cooling may have to be looked into. But this is secondary compared to getting the correct memory module that is rated to run at the desired FSB frequency and timing.
Some new mb, such as NF7-S rev 2, has wide range voltage for chipset Vdd, Vcore, Vdimm, so no hardware Vmod is needed. Otherwise goto the individual mb section, there are lots of info about various Vmod.
Putting a passive heat sink on the SB with good air flow over it should improve stability at high FSB oc. Adding a fan on the heat sink of the NB (if there is no) and SB would be even better if wanting to go even higher FSB (220+ MHz).
Memory testing
Test the memory module using memtest95 at the rated frequency and voltage to make sure it runs at spec. memtest95 is running almost on bare hardware, outside of Windows, so the testing can be done without some uncertainty and stability issues related to chipset drivers, video drivers and intensive FSB traffic, ...
Another nice feature of new mb is that FSB and memory bus speed can be run ASYNC with each other, for memory and FSB testing to avoid having uncertainty for both of them. Set one of them slower and oc the other as much as possible to find out their limit separately. E.g. set fsb:memory ratio at 3/4, would allow you to test memory at 200 MHz while running FSB for sure at 150 MHz, etc, etc, ...
Max FSB oc freq does not depend on the CPU per se, since the multiplier can resolve that, assuming you have an unlocked CPU, such as Tbred.
Summary
In summary, if you have good heat sink (e.g. SK7 or SLK-800), adjustable high CFM fan, good PSU w/ enough active current sourcing rails, good CAS2 DDR PC3500 memory, then they are sufficient to get the CPU and FSB to above average overclocking, within 5% of the full potential of your system.
The last 3-5% would be more costly and may require more experience and technique and possibly better components. But probably at that point, one would have already acquired enough know how to get to the max oc and beyond.