I would recomend a 750i or 780i SLI board for OC'ing.
Core 2 extreemes are going to be really expensive (more than an i5 now)
Try overclocking your Q6600 if you havent already.
The GTX 480 is at the same level with the Q6600, the preformance vs bottleneck isnt very bad with this combo.
If your board supports 8GB DDR3, get it done, memory is really cheap.