honestly you can get a 1gb set for a pretty good price these days. hell, you can find 2gb sets for just over $150. 512mb just really doesn't cut it these days. "midrange" pc's come with at least a gig now-a-days. memory prices have been greatly fluctuating the past 10 months or so, due to ddr2 coming out. now that it's pretty much the standard, the prices have somewhat settled. ddr is almost dirt cheap now. bad thing is that we'll have to go through it all over again when ddr3 makes it's face shown ><
i'm guessing you need pc3200 (ddr400)
1gb (512x2) dual channel:
http://www.newegg.com/Product/Produc...82E16820145440
1gb (one stick):
http://www.newegg.com/Product/Produc...82E16820145505
if your chipset supports dual channel i highly recommend getting that set instead. if it doesn't, either one will work about the same.