i can't overclock my 4670 512 MB GDDR3 form 750 to 778 Mhz.Tried ATI catalyst and Riva tuner separately. It works for some time but then my system stops responding after 2 -3 hours of gaming. My config is AMD Athlon 64 x2 4400+ @2.31Ghz,2GB DDR2@667Mhz, and 450W Standard PSU, WIndows Vista...