If its high end gaming/rendering system is what you're after then forget about ATI right now.NVIDIA is head and shoulders above ATI in GP-GPU/CUDA/Rendering department.... There's a reason why the Quadro range outsells the ATI equivalent FireGL 10:1
Having owned and used (and still owning) a number of 4870X2 cards, its not all bliss, their hot, very hot and sometimes even crash because of heat :/ Its rare but it does happen
Here is what you want to be getting.
GB EP45-UD3P motherboard
4x2GB DDR2 1066MHz sticks (UD3 board overclock ram very very well, if not the best in the market)
1x GeForce 280GTX
Intel Core 2 Q9450
800Watt PSU (make sure combined 12V rail amps are at least 58A)
If you have some money over or maybe an old 8 series VGA card throw it in there for Physics....
This rig will play the latest games at high res, do your 3d work with acceleration, provide physics support in supporting games, run cool, be fast and affordable.
That's a 12K well spent, you may even get some change from it.
With NVIDIA you will get, compatibility, reliability, Physics, Cuda, and great performance almost guaranteed.