If you've got all 4 cores running above 3Ghz you would probably be fine. A good way to check would be to overclock or underclock your current GPU to see what happens to your frame rates.
Run Fraps in one of the games you play and take note of your FPS. Then overclock your GPU (with Afterburner or something similar) and run again. If your FPS goes up, then you are GPU bound, and an upgrade would probably be worth it. If your FPS stays the same, you are probably CPU bound, and the upgrade wouldn't be worth it. You can also underclock your GPU if you are afraid of overclocking to see if the FPS goes down.
Alright, well it sounds like a GPU upgrade might be worthwhile for you.
Since the 7850 is a much better card than your 450, once you upgrade the CPU will probably be the bottleneck, but you'll see an improvement. And you can update your CPU whenever you have the cash, and you'll have a decent performance upgrade in the meantime.