I think people exagerate the need for a faster CPU. The CPU is not very often the bottleneck in a system, hell, just lately we started needing Pentium 4 2Ghz or so CPU required to play a game.
As for 3D rendering, more RAM and a stronger CPU over a faster video card.
The Core 2 Duo is plenty fine, and I would suggest it over the Athlon X2 unless you're going "budget".
/ttyl, church.