Use a name brand ps: corsair, antec, ocz, seasonic, enermax. All are pretty good. Ocz and antec are the best values.
Agreed on this. Also consider if you want to add future upgrades to go a little higher on wattage. Look for 600-700 watts with an 80+ rating. So it will be more power efficient at small and large loads. Many graphics cards now can eat the power, so if you every drop in a GTX580 or something, you don't need to buy a new power supply again later.
In my system, I originally had a 550w Cooler Master, which I found to suck. I have an AMD 965 3.4GHz quad core and a GTX465... the system always got stuck and did weird things. I replaced the PSU with a Seasonic M12D 750w PSU and the system is quieter and rock solid now.
So when buying a PSU consider at least one future upgrade on your system and give yourself at least 100w of breathing room from your current maximum requirements.