Hmm, I find that a bit odd, because you just bought that 450 to replace a 300 which you were told was not enough, then you get a 330 and it is enough. There might be another problem somewhere.
I find it odd that even a "cheap" PSU would be that far off in its rated wattage and actual wattage. It is possible that you got a bad one, but in any case it should have a warranty and I'd return and ask for a refund due to faulty parts. That Antec 330 should be plenty for your system and if you have further trouble you will want to look deeper into the problem. Maybe line voltage or surges causing problems in the PSU. Impropper grounding of the electrical system in your house could also cause problems.
I'm mentioning these things because you seem to have had to replace two PSUs in a short period of time. If it happens again I want you to have a little info to use to maybe help you track the problem down. You can buy a ground tester at a hardware store, or you can get a surge protector that has one built in. As for the line voltage, surges and spikes, you'll have to take up with the power company. Sometimes it is as simple as replacing a transformer, but usually you will have to get a power conditioner to fix it.