If ya have tight budget, I'd cut the RAM back to 8GB...the extra 8GB won't show a benefit in gaming.
Tho I think the upgrade route's gonna have a lot less options than most peeps are expecting, your best bet would be a Z68 Gen3 based board with SLI/CF a minimum 3 year warranty such as this Asus P8Z68-V/GEN3 and put the 2500k in it. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
The 850's will easily handle twin 6950's, 6970's, 560 Ti's or 570's. The factory OC'd Twin Frozr 6950's and 900 Mhz 560 Ti's are well matched as single cards .... but in SLI, the 560 Ti's top the CF'd 6950's buy 103 fps (862 to 759).
I realize the 16 gb of ram is overkill, but i picked it up on a daily deal for a solid price. I'll probably try and sell off 8 gig to recoup some money.
Can you elaborate on why you think the upgrade route is less desirable?
As far as power supplies, do you think i need to go with the higher watts since I"m only running a single card atm? I might go sli in the future after a couple years when the 6950 needs a boost but not right now. Would a 650 be sufficient now and then as well as save me electricity costs in the meantime?