I have recently upgraded my ATI Radeon HD 3650 to an Nvidia GTX 460. To do this I also bought a brand new power supply (Antec ATX 12V VP450). Everything installed okay, I installed the drivers (301.42). My Windows experience index went from 5.5 (with the HD3650) to 3.3 (with the GTX460). Now, I know that the WEI is not a good measure for this kind of thing, but I think that this is very odd. Games like diablo 3 and League of legends now run much worse than my old card? whats going on here?
Your power supply is at the minimum of what the GTX 460 requires, but let's assume it's sufficient. I would make sure you do a proper clean install of the 301.42 drivers. First, though, uninstall everything AMD and Nvidia...EVERYTHING from the Add/Remove Programs list. Restart, then install the new drivers and be sure to click on the "Clean Install" option. Only install the driver components you need. At a minimum you need the driver and the PhysX software.
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
I would have to agree with everyone else.Most likely a driver issue.Although that PSU is not some of Antec's best quality.And the fact that it's doing worse than the 3650 points to a power related problem,imo.
Try downloading DriverSweeper and be sure to clean out everything Nvidia and ATI if it is in fact a drive related issue.