Sure you can, but it will only improve things by a limited percentage, overclocking is a poor substitute for a more powerful card unless the particular card has a reputation as a good overclocker.
The overclock will be limited by two factors, firstly, then chips will only go so fast before showing errors, and secondly, the chips may get hot and then show errors or fail, possibly killing your video card.
So, when you overclock, if you notice any artifacting (strange things on the screen) reduce the overclock settings, or if the card is hot, reduce the temps and try again. If the card keeps showing artifacts, don't overclock it. Also, have a good explanation ready for the warranty guy if you bust it


Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&