It really depends on what devices you are going to be using. If you are using a couple hard drives, a CD-RW and/or a DVD-ROM/RW or whatever then you need to allow some power comfort. Also consider any devices that you will be connecting via FireWire or USB.
It wont hurt to "over do it". In most cases, you'll be fine with a power supply that puts out 400W or better.
I have a 420W in one of mine and have a bunch of external devices, 2 swap drives, a DVD, CD-RW, a lot of color-lit fans and a couple of UV tubes. I've never had a problem with power either.
In most cases, it's not the wattage amount, but the quality of the PSU itself. Make sure to buy a respected brand.