Sapphire 4870x2 and black screen?

I just got a biostar TA790GX 128 with 8 gigs of patriot 1066 ram , a phenom II 940, and a sapphire 4870x2. i started everything up and it all worked fine, when i installed the catalyst drivers and restarted all i got was a black screen. i have tried many different things, only using 1 chip of ram, doing a clean boot and installing the graphics drivers from the cd, i have also done a clean boot then upgraded the drivers from ATI, the computer runs fine with the onboard graphics.

when i do restart and get the back screen i can still hear windows starting,
i have also tried going to msconfig in safe mode then boot it with base video, which is what ati told me to do. i have a gpu that other people on the web have posted as working for them, i have a thoughpower 450 watt that is a dedicated gpu psu, and an antec 500 watt psu. see i dont have room in my case to put anything larger then most stock psu. please i am going crazy not being able to do much on the computer.