Guess it won't be so cheap but...
AMD Phenom II x3 720
Radeon HD 5870 (5970 if you can afford it)
MSI 790FX motherboard
A single, high-volume 4GB RAM stick (like the Ripjaws series)
I'm not so sure about how will a gaming card perform in the 3D Vray app but i'm sure it will not be so bad.
Why not check Tom's configs?? I'm sure you'll find something that suits you: