Use the following guidelines to choose a power supply appropriate for your system:
Above all, make sure the power supply you buy fits your case.
There are literally scores of brands of power supplies available, many of which are made in the same Chinese factories and simply have different labels attached to them. Most of those are of mediocre quality or worse, but some good name-brand power supplies are made in China. For years we have exclusively used and recommended units from two companies, Antec (http://www.antec.com) and PC Power & Cooling (http://www.pcpowerandcooling.com). Both produce a wide range of models in different capacities. One of them is probably right for your needs.
When it comes to power supplies, too much capacity is far better than too little. Using a 450W power supply on a system that draws only 250W does no harm; assuming equal efficiencies, the 450W unit consumes the same amount of power as would a 250W unit. Using a higher-capacity power supply than necessary costs a bit more, but has several advantages. The larger power supply generally runs cooler, because its fans were designed to cool the unit when it runs at full capacity. The larger unit typically provides tighter voltage regulation because it's not being stressed. And when it's time to add a faster processor or video card, the larger power supply has enough excess capacity ...