There could be a million dumb reasons that your computer is freezing. But you seem to think it has something to do with your PSU and GPU.
According to Nvidia's website your suposed to run a system with an 8600 with a minimum 350 watt PSU. Are you using a stock Dell 305 watt PSU? In that case that 305 watt number is total peak wattage across all rails (12v, 5v etc.) not sustainable power.
My motherboard lets me monitor my PSU voltages, but I'm guessing Dell wouldn't let a consumer do such a crazy thing. And you could get open up your case and use a voltmeter but...
Everytime anyone turns on a computer the PSU (and every other electrical component) gets weaker and weaker even if it's only .0000001 V per hour of use, it's the nature of electronics. Using a PSU that is stressed to it's max everytime you use it will make it weaker faster.
So I think it would be in your best intrest to buy a new PSU. Maybe it won't fix the problem, but maybe it will. And you can use it in a new system in the future if you plan to upgrade.
But make sure you buy a quality (at least "80 plus" rated) PSU.
Here is a list of recomendations:
http://www.eggxpert.com/forums/thread/101922.aspx