Just in case you need a second opinion, yes, you definately need to get a new PSU. I would say 420w minimum. Upgrading from that x300 to a 6800GT will be HUGE!!!!!!!
Unplug everything
Unscrew the 4 screws at the back of the PSU.
Rip it out, do w/e with your old PSU.
Put the new one in, screw it in
Plug everything in