NVIDIA GeForce GTX 580 to be 20% faster than GTX 480 ?
 

20% is a realistic improvement. Indeed, if Nvidia meet the original target for GF100 - 512 SP @ 750 MHz, that is alone worth a 10%-12% increase. Combine this with an improved GDDR5 memory controller and faster speeds, some minor tweaks, perhaps an increased TMU count (following a similar ratio to GF104) and 20% improvement is achievable. The other possibility is an expansion of GF104 to 11 or 12 SM with a wider memory bus and ROPs - which gives 528 or 576 SP. (Though this would affect compute performance) Nvidia's main challenge will be greatly improving power consumption - considering GTX 480 already uses upto 300W, which is the PCI-SIG limit. The GTX 460 is far superior than GTX 480 for gaming performance per watt, and perhaps GF110 will incorporate optimizations included in GF104.

http://vr-zone.com/articles/rumour-n...480/10116.html

And how much cpu power is actually needed to get the full 100% gpu power?

Core 2 Duo 3ghz+ for most games; to be safe, either Core i3/5 at 2.8-3.2ghz, or 2.6 Core i7 or equivalent. AMD's are good with X3 at 3Ghz.
in most cases you'll max out the GPU power before you hit a CPU wall, since you'll want to get the most from your new video card, this means enabling AA/AF and running at higher resolutions;
even with SLI/CF it's not very often that CPU becomes a bottleneck....