A Corsair CX500 watt should be enough, I'd go for the CX600 just for the eatra watts, though.
If you've already used the 330 watt PSU, you mave have already done damage to the rig if you are having problems, you won't really know for sure until you acquire a better PSU.
You shouldn't have more than 200 watts excess power on the 12v rail past the maximum, it could affect the capacitors. So if your total 12v rail for all hardware (ram, hdd's video card, network card, sound card external ports etc) is about 400 watts then that would calculate to about a 500 watt PSU. The only reason I'd go for the 600 watt PSU is for any excessive peaks or spikes that may occur and that there is a fair amount of excess available.http://www.dell.com/us/p/alienware-x51/pd