Dual Channel Memory (1 vs 2 sticks, sync vs async)
Many would say sync timing gives better performance in most MB, and dual channel is preferred for nforce2/P4 motherboard (MB). I think it's a bit more complicated, and depends on the memory speed (RAMDDR in MHz), compared to the FSB speed (MHz).
Assuming you found the top speed of the FSB, memory and processor of your system. AGP/PCI speed are assumed locked.
1. Memory speed is equal or higher than FSB. The max BW is limited by the FSB.
E.g. the MB can run at 200 MHz FSB, RAM is PC3200 (DDR 400). So you will get a max memory bandwidth (BW) of 3200 Mb/s. Actual BW ~ 3050 Mb/s
1a) SYNC with 1 stick: Can go a few MHz higher and tighter timing, may get 2-3 % better BW.
2a) SYNC with 2 sticks: May get 2-3% better BW (less overhead). Difference of 1a) or 2a) are insignificant.
For current AMD MB, there is little advantage to do dual channel since the FSB with SYNC memory can only take the bandwidth of 1 memory stick, unless you are using the integrated video. But a very different story when running in ASYNC mode with slower memory.
2. Memory speed is slower than FSB (e.g. running P4 with dual speed FSB, or reuse older/cheaper memory), so FSB can absorb most of the dual channel memory BW.
E.g. FSB 200 MHz, ASYNC 84% dual channel using PC2700 (DDR333), it will give a boost in memory bandwidth of 10% compared to running SYNC at 166 MHz. It is like running the RAM at 166 * 1.1 = 182 MHz. This is 92% maxBW w/ slower memory !!!
E.g. FSB 200 MHz, ASYNC 75% dual channel using PC2400 (DDR300), it will give a boost in memory bandwidth of 18% compared to running SYNC at 150 MHz. It is like running the RAM at 150 * 1.18 = 177 MHz. This is 87% maxBW. Best price performance tradeoff !!!
E.g. FSB 200 MHz, ASYNC 66% dual channel using PC2100 (DDR266), it will give a boost in memory bandwidth of 28% compared to running SYNC at 133 MHz. It's like running the RAM at 133 * 1.28 = 170 MHz. This is 83% maxBW. Very economical !!!
E.g. FSB 200 MHz, ASYNC 50% dual channel using PC1600 (DDR200), it will give a boost in memory bandwidth of 55% !!! compared to running SYNC at 100 MHz. It's like running the RAM at 100 * 1.55 = 155 MHz. This is 78% maxBW with slowest memory.
Dual channel can increase memory bandwidth up to 50% when the FSB data rate is 50-100% faster than the memory data rate.
Dual channel or single channel mode in nforce2 mb is not that crucial for overall performance when fsb and memory at running a 1:1 speed (or data rate). The difference is few % (for the same FSB, say 2-3%, maybe 5% for new NB stepping ?). Also single channel may let FSB to go a few MHz higher due to a smaller chance of 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 offset the performance advantage of dual channel, and the two is within few % (depends on overclocked FSB speed, motherboard, memory modules, NB stepping, applications), I think, for AMD mb. For some nforce2 mb that have integrated video which can benefit from twice the nforce2 memory bandwidth, since the bus between the video and the memory controller has 2x64 bit bus.
In summary, for AMD MB, if the RAM and FSB are comparable in data rate, running them in SYNC mode will deliver highest bandwidth determined by the FSB (8 * 2 * FSB Mb/s).
For slower memory (reuse older memory) running AMD MB, or Intel P4 whose FSB is twice as fast, the FSB can absorb the dual channel memory bandwidth. Running dual channel would deliver much higher BW.