artleon, in Vue 6 an Intel Core 2 system is approximately 15% faster than an AMD X2 / Opteron system at the same GHz. 

You can see benchmarks result here : http://www.cornucopia3d.com/forum/viewtopic.php?t=3344

A very good price / performance ratio is the Core 2 Duo E6600 (2.4 GHz and easily overclockable), but in fact every Core 2 Duo, from E6300 (1.86 GHz) to E6700 (2.66 GHz) is a good buy. I do not recommend the X6800 as its price is too high compared to the quad-core QX6700 (which is the best performer for Vue 6 rendering right now).