I assume you would want to be a 1:1 overclock meaning that you will overclock the CPU just as much as the Memory this will give you the best overal system performance boost. You could also increase the cpu by a factor 5 and the memory by a factor 4 for example in wich case your memory wont overclock at the same level as your cpu. Lets do some maths together
you got a P4C at 2.6 Ghz that means your multiplier for the CPU is set at 13 while the standard Fsb is set at 200
13x200mhz = 2.6 Ghz there you got your cpu speed
the system bus is quad piped wich basicly means the FSB x 4
4 x 200mhz = 800 Mhz
the memory would standard be running at 2 x 200 Mhz = 400 Mhz (PC3200 mem is set to run at 400 Mhz)
Say you will raise the FSB to 250 mhz wich should be a safe overclock with a good heatsink and a moderaltey good fan.
13 x 250mhz = 3.250 Ghz (CPU)
4 x 250 mhz = 1000 Mhz (System)
2 x 250 Mhz = 500Mhz (memory)
Here is a a choice you will have to make PC-3700 memory is set to run at 466 Mhz (FSB set at 233 Mhz) Pushing it to 500 Mhz is already a overclock on its own and you will need some good memory corsair and ocz come to mind to be 99%sure that your system can handle it. (you can never be 100% sure as there is no guarantee the memory will overclock to 500 mhz but those brands most likely will) You could also splash out on some PC4000 memory wich is guaranteed to run at 500 Mhz. As a bonus you still have some head room to take the 1:1 overclock a bit further should your CPU be a good overclocker.
If i was you I would get me some PC-4000 Mem, clock the FSB at 250 mhz and then graduately take it up little by little to see how high you can get without setting the Core voltage higher then 1.6V.You will have a safe overclock of around 650 mhz and the PC4000 will give you some room later on should you ever get better cooling. Settling for 233 Mhz FSB would be a waste of the potential of the P4 2.6 and the motherboard. Going for 250Mhz FSB on PC-3700 mem would limit your future overclockability.
My 2 cents, hope it helped a bit.
Almost forgot no matter on what memory you decide make sure it is dual channel memory. Its memory sold in packs of 2 and its optimised for working together.
