Thank you all so much for quick replies, thank you espically wildcat- you have been a huge help to me. This is by far the best forums ever and will be telling my clients about this.
This, but just to elaborate slightly - overclocking will shorten the processor's lifespan, potentially reduce stability and increase the amount of heat in your system.
It can be worth it if you need a bit of a speed increase, but if there's nothing you particularly require, you're just stressing the CPU for nothing.
Justy, if you want to rebuild a dell, think twice. I've tried, but without success. Hp dell and a few others have proprietary motherboard screw hole placement so you'll need a mobo from dell, and that'll probably eat through your $200. It sounds like you'd only be using the dell case and hdds, if that's right, you might as well start from scratch since a new mobo probably won't fit.