CPU: best value for money
Athlon 64 X2 6000+ (2 cores à 3000 MHz) -> $120
Phenom X4 9550 (4 cores à 2200 MHz) -> $190
Core 2 Duo E8400 (2 cores à 3000MHz) -> $180
Core 2 Quad Q9300 (4 cores à 2500MHz) -> $280

Chipsets:
amd cpu -> nforce 790 or nforceGF 8200 for hybridSLI
intel cpu -> X38 or X48 (don't take the P series - PCIe perfomance is worse)

GPU:
currently, there is no way around the GeForce 8800 or 9600 / 9800 series, $140 - $250, depending on manufacturer and memory size

RAM:
2 GB or more DDR2-800, any branded module will do, always take 2 identical modules, a good price is around $60 for 2 GB, good latency 5-5-5-15 (will run at 4-4-4-12 if it's quality ram)

+ power supply unit + case + 2x SATA hard drive (RAID) + DVD burner, i estimate the price at $500 to $700 for the whole system without peripherals like monitor, mouse, etc.