Although you don't mention what processor you use, PC2700 and even PC2100 (depending on your CPU) is more than enough for you today. I suggest getting PC3200 though, because later on, if you decide to get a 400MHz FSB Athlon XP processor, you won't have to upgrade your memory.
Also, from my experience I'd say go for Corsair XMS memory modules. I always used to go for cheap memory (of course money was the problem) but last month I decided to try Corsair 2x256MB PC3200LL (Twinx) and I can only say that the difference in performance worths the extra money!
I'm in agreement with <b>pisti</b>. Or for a future motherboard, I would imagine that PC3200 memory would offer a bit more. Nothin worst than throwing out old memory because it's preventing you from getting the most of your processor.
On nForce2 motherboards CPU and memory work together best when they run at the same speed.
If you have a recent Athlon you could try to up your FSB (and lower your multiplier accordingly) to run it in sync with the new PC3200 memory you want to get. Otherwise you may want to stick to PC2700?