PSU wattage isn't everything. You could have a PSU with 100000 amps on the 5v rail, but 1 amp on the 12v rail. I know, that just wouldn't work at all since the motherboard will draw over 1 amp on the 12v rail but it's an example. It also needs to be high quality too. I've owned some pretty cheap power supplies in my time, mostly bad, a couple of them are good mind you (only Fortron PSU's though). My list of decent PSU manafacturers:
Zippy/Emacs (They used to make PSU's for PC Power & Cooling until PCP&C could afford to build their own. Very very very good, but noisy.)
PC Power & Cooling (Mixed opinions on this one, some people say they are shit, but I've been using them for years with no problems. They can handle my insane overclocking very well. They make a PSU which peaks at 1kw
Nuff said. Incredibly noisy though!)
OCZ (They don't make their own PSU's, but they get them made up for them. Good stuff though.)
Antec (Ok, they outsource their PSU's too, but I've used them and have no complaints.)
Fortron Source Project (aka FSP Group. Very cheap, yet very very good. I have tons of these lying around.)
If you want more advice, I would hover over to
XtremeSystems and make a thread in the general hardware section, rather than posting here. Many of the people there have been building systems for years. I am there too, so there will be at least one familiar face there to help. [/pimp]