i have a 300W, is it enough to run a 7900GS 256MB 256-bit GDDR3 PCI Express x16 Video Card or something liek that???
Pull off the side of your case and take a look at the sticker on the side of the power supply. You should see a table which indicates the maximum current available on each rail (e.g. +3.3V, +5V, +12V). What's the maximum on the +12V rail?
You may be better off to get a new power supply, rather than look for a card that will work with your current one.