Depends on the games you play. At the moment as I understand it CPU speed is the most important factor after having two cores.
Overclocking your cpu would help but would probably require an aftermarket heatsink/cooler.
But I'm not that familar with games.
The X3 is worth trying first unless you plan future upgrades.
I believe at this time either the Athlon II X4 or the Phenom II 955 offers all you would need.
With an 890FX and DDR3 1333 you can run 14x250MHz at 1.375v with the IMC/NB at 2500MHz and 1.2v or less . That should give you an easy 15-20% on your frame rates. Your biggest problem is your power supply (assuming you are going with the HD6950 2GB):
Use 150w for the remaining system load (OC'ed)
edit --- I'm now firmly on the fence I think you can use your PSU and go with the HD69502GB but ...
Using PowerTune you will need to find a 'sweet spot' somewhere between 210-230w for your video card. You will most likely need a new PSU when you flash that second BIOS to a 6970 and boost the the volts a bit.
A new PSU would also give you some room for 14.5x250MHz at 1.4v.