I think a much better solution to rather than overclocking is to upgrade your system.

I am going to take a guess and say that your overclocking to run a demanding game, or program. In which case its most likely graphics related.

Thus you can start by upgrading your video card. Try something cheap like the Nvidia, 8800GTX 320mb. Its very good on performance, and is quite cheap..

The next thing is ram, your ram is way to small to even have vista installed on your pc. I would suggest 2GB of ram, 1GB for vista and 1Gb for your applications but atleast a minimum 1GB. So that means you need atleast 512mb more.

I wouldn't suggest overclocking unless you are ok with loosing your pc parts sooner than its intended life expectancy. If you must make sure you have proper cooling, water cooling is nice, but proper air vents and cooling should do. Since its the summer, you might need like an air conditioner in your room also.