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:http://www.tomshardware.com/reviews/value-pc-build,2506.html
Or maybe this config:http://www.tomshardware.com/reviews/build-own-pc,2503.html
I recommended AMD so it can be a cheaper computer, the MoBo i pointed out is like $170 vs more expensive P55 motherboards (around $250)