There will be a small performance boost, but you will most likely not notice much of a change in games themselves. For me the upgrade really let me run a lot more programs at the same time. For example, at the last lan we had, which had about 110 ppl attend, I was running my rig as a bf2 server, a cs server, and i was playing single player cod2 at the same time.
SO in conclusion, you will get a performance boost, but i can pretty much guarentee that you wont see much improvement in gaming.