Moving on

So with christmas coming around Ive found myself wanting to upgrade again.
I plan on going to an mATX i7 system sometime next year but i want to atleast change my graphics to make the process a little smoother.

Ive loved my 8800gt Sli setup but since I wanna go matx I want a single card solution. Now Ive been on the forums long enough to not have to ask for advice on stuff like this (usually google solves most problems) but Im just a bit lost on the performance of these newer cards.

Basically, should I replace my Sli setup with a single 5850 or 5870.
Will I see improvement or pretty much the same thing for less of a power strain on PSU?

All Ive been playing lately is Dota, CS:S, assassin's creed(trying to beat so i can play the new one), Heroes of Newearth. But there are alot of new PC titles coming out and I wanna beable to play some of the higher end ones.

Is it worth the cash?, Wait for fermi?, Go nvidia instead?

Opinions, thoughts?

Thanks in advance.