A 7870 would bottleneck your cpu quite a bit. I would consider upgrading that if you want to get a new gpu. Also whats your Motherboard, cause 2 6750's in crossfire are amazing, equal to a 560ti, still would consider upgrading that cpu though. IF you want, you can get a 550ti for around 120-140 or a 7770.
My motherboard is a FOXCONN2AB1 I believe.
And can i just buy a single 560ti? is it short and wont bottleneck my cpu?