For gaming a Core i7 2600K will be faster and also it does seem even a GTX580 will be slower than a dedicated AMD or Nvidia card for most work too. I would consider the dedicated cards very important for actual model building
TBH than for the actual rendering machines. I wouldn't put all your eggs into an Nvidia basket though as you really should check out the newer AMD cards. They tend to be better value overall.
Anyway,here is a useful article to read:
http://hothardware.com/Reviews/AMD-F...phics-/?page=5
A Quadro 5000 is around £1656 on Scan. The V7900 costs around £584 and the V8800 costs £978. The V4800 costs around £120.
I do know that there are some CUDA plug-ins which are meant to accelerate rendering though so perhaps maybe the rendering machine would be where the gaming card will be more use. With your £3000 budget you could build both machines. You could have a V7900 and a Core i5 2500K in one machine and a GTX580 and a Core i7 980X in another.