It's always a good idea to choose a PSU that has a bit more power than you need, thus allowing for future expansion. Picking a PSU can be a bit of science though - the raw wattage of the PSU isn't the only factor to take into account. The two primary factors are the total amperage of and configuration of the 12v rails as well as the reliability of the power it's distributing. It's a sad fact that the majority of PSUs on the market are of relatively poor quality, carry a high risk of early failure, don't provide the power they're rated for and can potentially damage other components when they finally do die.
If your computer is a entry to mid-range computer, I wouldn't worry to much about the particulars. Generally, any quality 450-600w PSU should suffice for a computer in this range. If you have anything like SLI, a high-end processor or a lot of hard drives, then you will want to do a bit of research and determine what your power needs are. Regardless of the PSU you do end up choosing, I would highly recommend reading as many reviews as you can find. One resource I would recommend is
JonnyGuru's site. He does some of the most reliable and thorough PSU reviews on the internet.