I just switched from two Radeon HD 4870 1gb cards to two Radeon HD 5770's, and I prefer the newer 5770's. More power effecient, and perform about the same and sometimes better than the 4870's. Plus someday if you decide to go with Crossfire and throw another in, the 5xxx series cards scale better.
Those BEnches were also run with the first drivers released for the 5xxx series and there is already a new driver out that improves the performance of the 5770 and more driver issues will be fixed as the cards are out for awhile (remember most card drivers when the cards are released do not perform that well) so the difference is already less than it was and since the 4870 has reached its end of Life cycle it is going to be hard to find them in awhile so if you do ever decide to go crossfire it will be more expensive for older tech. (as cards become hard to find they increase a bit in price while the newer 5770 will be dropping in price at the same time !!) - Also within a few years DX11 will become the standard and many games will no longer play on older DX10 cards so you will wind up having to buy again to be able to play the newest games.
IF you look at the release notes for the newest driver it mentions :
"The following performance gains are noticed with this release of catalyst 9.12:
3dMark Vantage - overall performance improves as much as 9% on ATI RadeonHD 5800 and Radeon HD 5700 series products. GT1 - Jane Nash performance improves as much as 15% and FT4 - GPU cloth improves up to 15% "
What is the native resolution of your monitor, what cpu is in your system and what are the specifications of your power supply?
These are all more important questions as to what card you should buy than a pure performance comparison.
Yeah, 4 gigs of ram and a better brand of PSU.
Also are you really spending almost as much on a cooler as your CPU? That's just... odd. Get an Athlon II x4. Cheaping out on the CPU while spending that much on a video card and a motherboard with 4 PCIx16 slots makes no sense at all. If you insist on MSI the 790X-G45 should be much more reasonably priced.