am using an ultra X4 modular 750w on an asus mobo 8gigs mem, 460 gtx w/twin power connectors, also check your mobo specs your Pci-e my be outdated, todays specs are 1.5-2.0 slots- Peace
try upping your power supply to 750 w, mimium, over 950 w if you use twin cards, also up the current memory, todays games need at least 8 gigs of motherboard mem