Go read ceewi1's PSU guide. Here's the link:
ceewi1's PSU Guide
That's a superb guide, but not one I can take credit for - you can thank Praetor for that one!
had a bit of a read. mines only a 350 watt at the moment, is it basically the bigger the better or can i overload my system?
There are two issues at play here. One is the physical size of the PSU, whether it will fit in your case. There are a few form factors, e.g. ATX, Micro-ATX and others. In some cases Dell will even sell systems that don't use a standard sized PSU. In general, though, you'll probably need an ATX PSU, unless you have a small form factor case.
The other issue is the amount of power that can be put out, and in that respect there's no limit - you can't overload your system. As has already been mentioned, make sure you look for a quality unit, and one with sufficient amperage on the +12V rail(s), not just wattage.
OEM PCs (at least those from major manufacturers) will generally come with a PSU that is sufficient for the system, but without much upgrade room. If you're planning on upgrading, or need a new PSU for some other reason, please post your budget for the new PSU, your system specs and the model of your PC, if it's from a major vendor.
If you're looking for more general information, there's a lot of it at
http://www.computerforum.com/90118-useful-psu-case-guides.html