Consider bypassing the quad-cores and maybe go with the new Intel i7 CPU. The CPU is 4 cores with 4 Hyperthreaded cores.  The new Hyperthreaded cores are much more efficient than the old P4 Hyperthreaded cores.

In essence, you end up with an 8-core CPU -- maybe the equivalent to 6 real hardware based cores.
Some rendering comparisons with 3D Studio Max 9:

*Core i7 920 (2.66 GHz) = 0:31
*Core 2 Quad Q6700 (2.66 GHz) = 0:43

And with Cinema 4D Release 10:
*Core i7 920 = 1:44
*Core 2 Quad Q6700 = 2:18

The i7 motherboards also support 12 GB of 3 channel RAM.
Just don't forget to use Vista Business 64-bit or Ultimate.

*Complements of Tom's Hardware: http://www.tomshardware.com/reviews/Intel-Core-i7-Nehalem,2057.html