Ever since I upgraded to a nVidia 7800GS and 2GB DDR400 RAM, my computer just randomly shuts off (at least once a day if I have it on the whole day). I can still hear the fans going (on the processor and video card), but the computer has shut down and I need to switch it off and wait a few minutes, then start it up again.

I was talking to a friend, and he said that perhaps my PSU is not powerful enough (I have not checked how many watts it has), and that I should go and buy a 450watt PSU as that's what is recommended for the video card.

Do I need a new PSU? Will any fit into my computer? Are they easy to install?