I looked it up a bit and came up with these:
http://www.tomshardware.com/forum/395067-33-crossfire-7850-user-review
http://www.pcgamer.com/2012/11/19/hd-7850-1gb-crossfire-benchmarked-this-generations-best-multi-gpu-buy/
Honestly, it seems like the only limitation you would face by CF 7850s would be the 1gb vram and that you might need a new PSU 750.
A good choice for the psu would be this:
PCPartPicker part list /
Price breakdown by merchant /
Benchmarks
Power Supply: XFX 750W 80+ Bronze Certified ATX Power Supply ($79.99 @ NCIX US)
Total: $79.99
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-04-03 19:32 EDT-0400)
But you may be able to squeak by with the 650w psu depending on how old it is.
Personally I would try to get a 280x or the 7970 mainly due to the 1gb vram and stability of a single card vs dual cards