Mem for E6600 for P5B

I am going to get 2x1GB for this setup. I am choosing between PC4200 and
PC6400. Is it better to go for 1:1 (PC4200) or for higher performance
(PC6400)? Also, does it make sense to get a matched pair? I am not going to
overclock.

No overclocking? You could get PC2-4200 RAM and run the 1:1 memory ratio. I would be inclined to recommend at least DDR2-667 though so you can run the 4:5 ratio for a little extra memory bandwidth. Since DDR2-800 is not much more, you might consider getting that for possible future overclocking or upgrades.

If you are 100% certain that you will never ever overclock that rig then go ahead and get the cheapest, but still quality, memory.

Yeah if you arent ocing get a matched kit of anything cheap from a reputable company.