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I'd suggest getting an Athlon 800 - 1200 save your money for 256MB RAM it's pretty cheap these days. The Ultra is quite a bit faster than the regular GF2 ~50% I think but costs about $400 so you might get the MX and wait for the NV20 which is the next Nvidia chip. Just don't get a cheap motherboard and get 300Watts of power if you're going to upgrade to multiple drives. Check out http://www.tomshardware.com/ there's a new article that'll break it down for you.
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