You have to ask yourself, how powerful do you want it, though here is a clue.
Amd Athlon 64X2 4200
gig ram
80 or 120gig hard drive
Motherboard that supports processor
Nice looking case
Min 500watt psu
grforce 6200, or 6800gt
Research them up and find out a price for all of them and base it off that, if it comes under $900 go for something higher like a 160 gig hard drive