In respect to how much memory to get, that depends on what type of user you are. I would suggest nothing less than 512 under windows XP. I multi-task a lot and I run chem. emulation software, so I went with 512. And on top of that I still find myself bumping up my virtual mem settings, so I think I may even be purchasing mem in the near future.
Now, whether to buy 2 stix or one is a different issue; it's a trade off. If you buy one stick you get more stability. If you purchase 2 stix, you have a backup in case one goes bad and you have to wait for a replacement. Also with 2 stix, you leave yourself ready for the new dual channel ddr boards that are starting to appear (on a side note, if you're interested in one of these new dual channel boards, I would wait until the second generation of them comes out or for the bios to mature for them. Current preliminary tests don't show the performance I had hoped for with the new chipset. The mem controller on the GA-PE667U is very efficient and, if you've been paying attention to the latest news, you'll see that this chipset stays very close to the new chipset, and that's not considering the speed I'm running my system out now).
In reguards to latency, yes, CAS2 is better than 2.5 or 3.0. Most of the well known companies have modules that will run at 400mhz CAS2 now. You would do just fine with Corsair or Mushkin. Just to give u an idea of how much timings affect performance, at 350 mhz running at 2-5-2-2 gave me 45megs of additional bandwith over running 2-5-3-3.
Enjoy the hunt...