If you do a lot of rendering get yourself a quad-core, preferably an i7 as it allows multithreading. The Core i7 will
kill Keyshot
I have a 1st generation i7 (Lynnfield) and its a monster; the later generations (Sandy Bridge, Ivy Bridge and Haswell respectively) only get better and better. Anyway multithreading (or multiple cores) really show when it comes to rendering, not so much with GD. However the number crunching and memory capabilities of the i7 will make your Pentium Dual-Core seem ancient.
Graphics card-wise... get a decent gaming card i.e it must support directx. The modern cards already support directx 11 so you'll be fine with one of those. Avoid quadro cards as they support openg,l not directx. I opted for a Nvidia GTS250 and it does a fantastic job without breaking the bank.
Try to avoid AMD. I remember reading way back that some users had issues with (then) Alibre running on AMD hardware.