new card advice needed

Good morning folks,

Im going to be starting a new build soon, consisting of a z77 mobo, i5 2500k(o/clocked to 4.5) 8 g of samsung 1600 ram, nh-d14 cooler, now im just debating what card to get myself, - and without wanting to start an nvidia/ati war, what is the card to use for the moment? - i might stretch myself to a 580 gtx, what do you guys think? i read somewhere that nvidia is more suited to fsx than what ati is - something to do with ati has faster shader processors, but nvidia had better ??? i cant remember what it was.

Also, im going from xp32 to windows 7 64, is there any issues i should know about prior to getting my o/s?

regards

paul