Generally speaking your game performance will be limited by your 6770 long before the CPU becomes an issue. The 2320 is the slowest of the bunch, but it comes with better integrated graphics than the 2400 and 2500. Since we are talking about the non k version it probably isn't worth the extra money for a 200 MHz bump over the 2400. Even if you got a second 6770 you still wouldn't likely hit a CPU bottleneck with a 2400.
core i5 2500 (non k) has higher clockspeed than 2400 and 2320. in some situations, it will lead to the 2500 delivering slightly higher minimum fps than the other two.
with 6770, a core it 2320 or 2400 will suffice.
if you can afford a 2500, pay a little more and buy a core i5 2500k + cpu cooler + z68 motherboard for overall better gaming performance than 2500, 2400 and 2320.