How well will my CPU run games?

Hi mercer, can you please write down FULL specification?

CPU: athlon II 645 x4
Memory: ???
GPU: ???
PSU: ???

Once you provide these info, then the community can provide more help.  
Without it, we can't judge whether your system can cope with BF3 & Crysis 2.

Overclock.

well, I have 8gb of ram a 600w ocz psu and my gpu is a 6670 but i only really want to know if the cpu can perform as I am upgrading the gpu soon, but i want to wait until kepler is released. Also because of my mobo I can’t increase the voltage higher than 1.4v so the best stable oc I can get is 3.4ghz.

Your graphics card is good. But, you could upgrade it just a little.

Second, your memory looks big enough. There is no need to upgrade that.

So yeah, my conclusion is to upgrade your CPU (Recommended for OC beyond 3.4GHz) + GPU (if you want better FPS). However, that is your choice and you may need to get a new motherboard and PSU just to keep GPUs + CPU in check.

say if i got a 560 ti or a 6950 would be cpu hold be back?

I heard that a lot! So, in short term, it will hold back if you got Duo Core CPU, Not Quad Core (like yours). So, you are safe...

There will also be no conflict if you decide to upgrade your GPU 560 ti / ATi HD 6950.

Also, I had a look at my charts. Your CPU is running @ 3.1GHz, right?
If so, then it's 3.1 x 4 = 12.4GHz. If you overclock it (which you wrote earlier), then it's way enough...

So yeah, upgrade your GPU and leave your Memory + CPU as default. No need to dump more memory and change motherboard.

im not going to change the mobo until i am upgrading to i5, i have being looking at the 6970 and at £220 do you think that it is worth buying? or should i get the 6950 for £170 and unlock it?

Many have suggest to buy Ati 6950 and unlock it. However, it's not a safe root. There has been plenty of user failing to unlock it or brick their GPUs. Therefore, my advice is buy ATi 6970 at around £220 and play it safe. Also, if you are going for intel i5 near future, then please look at intel motherboard (1155 socket). That is the latest which is a few step behind x79 motherboard.