You have plenty of power to run any single (processor) card you want so if you can afford it, go with the top.
Nvidia 580 GTX or AMD 6970 are the top cards. However, if you upgrade your mobo, you'll need a new power supply to SLI GTX 580, so take that in consideration, but I don't think you'd need to SLI for a long time, so it's kind of inconsequential.
What resolution to you play at? Also, though price is no issue, the Nvidia card is $130 more than the AMD card when comparing flagship cards.
Well iam willing to buy the best available for now and future use "Or maybe there is better GPUs releasing soon and i should wait ?"
Though its not my intention to"Buy expensive because expensive" rather it is " Buy expensive because performance". So if you think a cheaper AMD is better than a more expensive GTX please tell me, Iam all for performance.
Also the PSU is no issue too. if I need a better one in the future for Crossfiring/Sling ill buy one.