Well.. . I just got this computer in through the mail and installed windows 7 on it for the first time.. But the video card it came with isnt really working so well.. i had to put my old one in from my last computer... but lets skip to the chase...
Its a RADEON HD 4870 512.. I plug both power supply 6 pin plugs into the end and slide it into the motherboard slot and bolt it into place... And start the computer up... The Moniter stays black as the computer boots up... The computer still passes the post startup thing. But nothing displays on the moniter AT ALL. The moniter is an ACER of some sort... Flat screen.. using the white hookups to plug into the Vcard... But when i put the other Vcard in it works just fine.. I dont smell anything on the radeon but it isnt working.. Perhaps im doing somthing wrong..? any help?
It could be the power supply -- pre-built machines often come with crappy ones. Although if the 4870 came with the machine, you'd think they would know to give it a good enough PSU to run the card, duh.
Was Windows 7 the operating system that came with the machine, or did you install it yourself? If you did it yourself, there could be some kind of driver issue at work, either with the card or the motherboard. Try updating the BIOS and video card/motherboard drivers to the newest versions from the manufacturer's site, and see if that helps. That's pretty easy and you may get lucky and solve the problem. If not, it's time to start looking for hardware issues.
edit: also, if Windows 7 was not your original OS, did the 4870 work with Vista or XP or whatever it came with? That could go a long way toward determining the problem.