I also overclock my system.
I have a:
Athlon64 3200+
1Gb Ram
6800 GT @ 425/1150 (past ultra)
Never had a problem with any games. Your overclock probably isnt causing it unless you have crap cooling. Have you made sure your nvidia drivers are up to date? Try the 77.50 drivers from
www.guru3d.com
Also is your motherboards bios up to date & all your motherboard chipset drivers? 4Gb of ram will slow your system down, use 2Gb max if I were you. With 4Gb you're limiting your overclock & also it'll run slower. The more memory you have the longer it'll take to access. Get 1 or 2 Gb with good timings for best results. The only game that benefits from 2Gb over 1Gb at the moment is the battlefield 2 demo!!