If you are talking US prices then you can build a pretty nice rig for $1500. It won't be top of the line but it will be pretty powerful. That gets a 3700+ CPU for $300 (San Diego core=crank that baby up), a 7800GT for $384, 1GB of Corsair RAM at $124, a 500 watt PSU at $90, a A8N SLI Deluxe mobo at $164, Thermaltake Tsunami aluminum case at $122 and a 160GB SATA HDD at $80, a DVD/CD RW $38 puts you right at $1300. That leaves $200 or so for a monitor. You'll also need an OS which will cost around $85 and keyboards and mice and such but you may have those items already.
I'd recommend 2GB of RAM at $300 and a SLI ready PSU at around $166 for the Seasonic-12 but you don't have to have these right away.