I'd suggest just not going crossfire with that power supply and get a 6870. I'd think the max you'd be able to go with be 2 6750 and thats MAX, not good to cut it so close, performance wise 1 card would be pretty even with the 2 cards. I don't think it would be worth it to get 2 cards.
I think you should just get as much for your budget now and buy a 6870 or better, also what resolution do you game at?
1280 x 1024. Looking at going higher. that is one reason I am looking a graphics cards. looking for HDMI out, getting a better monitor. This motherboard is the backbone to upgrading the entire PC step by step. CPU and RAM good for now, trying to take the first steps for the other components, until I get a really good computer.
a single 6850 would be good now for that res, might even be a bit overkill.
a 6870 can play basically everything at 1080p with good settings
I'd say get the 6870 if you will upgrade to 1080p and don't worry about crossfire until you start to not be able to play games well and have upgraded everything else such as the cpu. It is a giant upgrade to what you have currently.
well you have a pretty balanced system with a 6870 and you'd have to upgrade your cpu to have a substantial effect.
only really good cpus for gaming today are intel so you'd have to get a new mb and a new cpu which could cost a lot.
I think you will be good for a while on this, you'd have to upgrade to an i5 for much increase in performance. upgrading to 2 6870 might also become a bottleneck for your cpu. you can always try to unlock your cpu into a quadcore or get a quadcore phenom which will help with that but the difference could be minimal while gaming.
I say spend the extra to get the 6850 but either will be a good upgrade from your dual 4670s and you can CF the cards latter. While it's true that your current CPU might limit two 6850s in CF it would still be a decent increase over a single card and you can use them latter when you upgrade the rest of your system.