Yeah actually
First thing your gonna have to do download coolbits (if you havent already). Coolbits is a registry edit that enables overclocking in Nvidia cards.
Second thing is now go into card settings > Clock settings (not exact name) and there should be 2 sliders: one for core and the other for memory. (If using coolbits oc in the third tab not second). Now first just work with the core. Move the slider about 5-10mhz and then go into 3d mark 2003 or ati tool (see bottom) and check for 'snow' (or i get little green blotches) if there are some then ur card is oced to high also check if it locks up if so back down about 5mhz and recheck. If there is no snow then you're good to overclock further. So repeat by increasing another 5-10mhz until you reach your maximum with no 'snow or artifacts'. Also, once u have reached the maximum it is a good idea to check your temps.
Now to the memory, firstly return your core to stock. Now move the memory slider about 5-10mhz and check in 3d mark 03 for any artifacts. Repeat until you get artifacts.
Now return your core to the oced speeds u had before in order to make sure the oced core and memory work together. Run 3d mark 2003 a few times to see if you have any artifacts or any sort of problems. It is also a good idea to play an intensive 3d game for bout an hour to check for any signs of problems.
If you get artifacts or other problems then back down both the core a memory a bit. Keep doing that until you get the most stable speeds.
With that done you've finished. Everything has worked out good and you have succesfully oced your card. If you want to oc further then you will have to buy better cooling. Eg. Water cooling, VGA silencer...etc and some ram sinks if you do not have some already.
mines at 564/1160... if you can get it higher than that without getting artifacts, then ill stare at you with a very evil look, and steal your secrets...
What i did was instead of going through 3dmark03 EVERY SINGLE TIME, i got ATI Tool (
http://www.techpowerup.com/atitool/ATITool_0.24.exe) and ran the Check for Artifacts thing for 45 seconds, if none were found... i ran 3dmark03...
Post your score and your clocks when your done... im curious to see em
How high are your OCs? 
i got my 6600GT up to 561/1140 with basically stock cooling... im getting 3779 in 3DMARK05 now 
