About to give up on computers and graphic cards!

Just got my new card today, a Radeon HD 7850 2gb. I installed it, turned computer on, nothing but a black screen, no error beeps or anything but the computer loaded into windows. WTH gives? I thought my PSU was a problem late last year but I've recently had it changed so now what? My onboard is the only reliable graphics I have and its a Radeon 3300. Do I need to go into my bios and disable the onboard for the new PCI-E card to work? I already have PCI-E set in bios to be the main graphics. I dont know what i'm doing wrong, are both PCI-E slots bad?