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prices are from Newegg.
CORSAIR XMS 512MB PC3200C2 --- $112
KINGSTON HyperX 512MB DDR400 --- $100
CRUCIAL 512MB PC2700 --- $71
Without pricing, the XMS is a pretty obvious choice.
I decided that I'd rather have a gig of slightly slower ram, so I went with 2 sticks of the Crucial. The price must have gone up recently, because I paid $67 a stick. I also seemed to luck out a bit, as I have them running dual channel on an 8rda+ at 200MHz FSB @ default timings of 2.5/3/3/7