I feel like every time that someone builds a computer with a mid-range card with the intent of CFX later, everyone always is against it. Personally, I think if you can find a 7770 cheap, CFX would be a great option. I'm going to be looking for a second 6950 soon to CFX with the one I have.
I had an older E8400 with a Nvidia GTS250 in it. Found on TigerDirect the same exact video card for like $40, so I bought it and am running it, I can actually run modern games now.
It would definitely improve your performance. Of course, one card is better, but if you can get two cards might as well.