Personally I'd say with an OC 7900GT you would have more to fear from the Amp load on the +12v rail than the total wattage.
I killed a Thermaltake 420w with an overclocked Athlon 1800+, 512MB, 2x HDD, and an FX5900XT.
Their PSU's I wouldn't personally recommend. I have had Antec for my last two and won't be changing anytime soon.
Do yourself a favour and get a NeoHE 550w. Have piece of mind that you have more headroom in terms of power (particularly if you want to OC your CPU too), your PSU is quieter, and that it is more efficient (save on those leccy bills).