It's not only the power in watts that counts, but more the quality, read stability on the various 12v and 5v leads. Antec has very good reputation and I bet an Antec 350w can easily outperform generic 350w powersupplies.

About the power question, with todays high performance graphic cards a good powersupply is essiantial, those 6800's and X800's are real hungry. For other purposes everything around 350 or 380w is more than enough.