Well contact Dell, it should still be under warranty if it is only 3-4 months old. Tell them that your computer is struggling with power issues and ask what they can do to sort it out. They will probably just say that it should be fine and give another month, but you never know, they might actually value you as a customer and do something about it!
It just strikes me as strange that Dell would put a 300W PSU into a computer that is running a 6800GT??But then I shouldn't be surprised..... it is Dell
I mean it doesn't take a genius to read the specs of a 6800GT and realise that 300W just isn't enough. I bet they've used a cheap PSU aswell, one with very little amps on the 12V rail.