Is that budget including what you already have? Do you need a monitor, keyboard or mouse?
Here's the main parts to start with:
CPU: i5-750 $200
Mobo: Asus P7P55D-E Pro $190
RAM: Corsair XMS3 2x2 GB 1600 mhz CAS Latency 7 $110
HDD: Samsung Spinpoint F3 500 GB $55 (or 1 TB for $90)
Optical: Cheapest SATA DVD burner $25
GPU: HD 5850 $300
Total: $880. That leaves room if you need the monitor and extras. If you don't need them, get and HD 5870 for $100 more and a 64 GB SSD (total of $1,180).
No Monitor Keyboard or Mouse needed. Thanks