I have a similar problem on a similar motherboard.
MA790X-UD3P + newly purchased Gigabyte GTX460 1Gb OC.
After much swapping of components and testing, it appears that my MB is the culprit. If I put the GFX card under any sort of load (games/benchmark or even video playback) the system freezes (although audio will often continue. I never really had much trouble with my HD4850.
I have tested the GFX card in 2 other PCs, and it is perfectly stable. I have tried 3 differenct CPUs, different RAM, a fresh isntall of Win 7, Windows XP and it still points to the MB.
Power supply is a Coolermaster SilentPro 700w, which has been tested in a friend`s PC with the GFX card.
The GTX460 looks good when it`s working, but I only get a few minutes of gameplay if I`m lucky.
Currently limping along with 4Gb OCZ Plantinum DDR2, Coolermaster 700W, AMD Phenom II 550, Win 7 64bit and a couple of Gigabyte boards that don`t seem to be happy with each other !