Hi! my experience has taught me that when it comes to 3d art and games the 3d card and the ram memory are the two most important parts in the computer the processor is of course vital but you sholud spen more money on the graphics card and the ram.
processor: amd 1.4 GHz
ram: 512 minimum
graph card: Ati , radeon series 9000 pro or above. good quality and relatively cheap, I know you probobly think that a nvidia card would be better but check the price tags and read the hardware tests.