Samsung (
ONLY true Samsung chips on a true Samsung PCB)
Corsair XMS
Mushkin
Crucial
Kingston
TwinMos
Geil (fairly new and not completely proven, but I've had great luck with them)
OCZ (used to have some problems, but seem to have completely redeemed themselves)
In order, those are my picks from best to worst RAM. The Samsung is sometimes hard to be completely sure of, so I usually get Corsair (NOT the ValueRAM). While OCZ is last, they could quickly move to first as long as I continue to hear nothing but good things about them. Their prices are good, and as long as the quality is consistent (and so far, as long as they've been under new ownership, it has) they'll become the RAM of choice. From Samsung to Kingston, though, you really cannot go wrong. All are really good quality.
The only reason to buy PC3200 RAM (though not a bad reason) is so that you are pretty much assured of running at PC2700 with the most aggressive timings. However, most of these (most definitely the Samsung, Corsair XMS, and Mushkin) will be able to run at PC2700 with the most aggressive timings, especially if you're willing to raise the DRAM voltage a little. The other reason to either buy high-quality PC2700 or PC3200 is that running it at PC2700 with the most aggressive timings won't stress the RAM as much. If the stick is stressed and runs hot, it could lead to possible instabilities and a shorter life of the stick.
I would recommend buying two sticks if you don't mind the extra cost. If you get any board with dual-channel DDR in the future, you'll need two identical sticks to take advantage of it. Also, you'll never need more than 512MB of RAM unless you do a lot of video encoding or CAD work.
Edit: Let me rephrase my first sentence. Those aren't my picks from best to worst, they're my pick from best to worst of brands I would actually buy from. I don't think there is any other brand I'd consider buying. Don't buy generic as you could get burned. I don't mean to say that Geil or OCZ are the worst sticks of RAM out there, they're just not at the top of my list as the RAM that I'd buy. However, for my system, I just purchased 2 sticks of Geil 256MB PC3500 Ultra Platinum from Directron (
here for $93) and I'm very satisfied. Just make sure you check the RAM when you get it, because they shipped me one stick of Platinum and one stick of Ultra Platinum; not much difference, but it's not what I paid for either.