Uninstall the graphic driver as suggested and run Driver Sweeper from Safe Mode (F8). Have it find and remove any remnants of Nvidia, AMD, and ATI graphic
drivers it finds. Reboot and install the latest driver pkg for your card. Run CCleaner to clean up your system of resource wasting crap and registry errors.
Driver Sweeper: http://phyxion.net/item/driver-sweeper.html
Here's a PSU calculator you can use for your system, but I think the Antec 450W is sufficient.http://www.thermaltake.outervision.com/