Yeah, the 945 would be nice then match it up with the 5770.
The 4870 was a good card but it outdated and its getting hard to find. plus with the 5770 you can run DX11 and it only $10~20.00 more than the 4870.
One more thing
Before you switch out cpu's update your BIOS!!
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PII 1100T (Extinct) Asus M4A78T-E (should be Extinct) 4Gb of Corsair XMS3 1600Mhz DDR3 (Going strong) 2x1GB 5850's (On their last lags) Corsair HX850W (Awsome) NZXT Phantom case (too ***** big)