koala

A 650W from a good quality manufacturer like Corsair, Seasonic or Thermaltake would be ideal for a 9800GTX+, but before we recommend a new PSU, let's check your Raidmax's output under stress.

Go into BIOS and note all the temperatures and voltages. Then reboot and run Everest or SpeedFan in the background while playing WoW fullscreen. When the game starts to lag, alt-tab back to Everest/SpeedFan and take a screenshot of the temperatures and voltages so we can compare the idle and stressed readings.

Remove the side panel from the case and blow out any dust with a can of compressed air, especially around the fans, heatsink and card slots. Make sure all fans are spinning and unobstructed by cables.

Repeat the volts/temps readings in BIOS and Everest to see if cleaning the case has reduced the temperatures and improved the PSU's +12V output to the graphics card. Cleaning and checking the fans might be enough to fix your problem. If not, we can look at other options.