I used an MSI Nvidia 8400 GS with 512ram and could play crysis on low spec just fine. without lag. Call of duty 4 runns well also. this is not a great gaming card but if you don't play crysis or bioshock, you should be fine. I just upgraded to an ATI 4850, and i must say, a better graphics card is a must for gaming, but only if you're into it hardcore.
Overclocking is possible throught the Nvidia control panel that you can download off of nvidia's website. it has an auto tune feature that will automatically find the optimal setting. it won't really boost performance that much, but it's noticable.