The inverter needs to be a little bigger than the load. For example, if you PSU needs 400watts RMS, then you need bigger than a 400watt RMS inverter, to be safe.
However, it may be that the 400watts rating of your PSU is peak, not RMS, which then, your inverter would be ok being bigger than 400watts peak (200 watts RMS).
If you don't know, then you can always search, or err on the safe side and use the bigger inverter.
All that said, just because you have a 400watt PSU doesn't mean anywhere near 400watts is being used; that is just the max it can output. Is the inverter 330 watts RMS or Peak? If the 330watt inverter was 330watts RMS, then it should not have fried unless it's fan went bad or there was some other problem, as the PC you described isn't likely going to pull 330watts RMS.