My Dell GX260 won't display any video

I picked this up used about 6 mos. ago minus ram and hard drive. I installed a small hard drive and 256Mb of memory to test it, and it worked fine. After that I put it up for awhile until I could get more ram and a bigger HDD. Yesterday I installed a gig of DDR400 and an 80Gb Seagate, and when I turned it on, it had no video whatsoever. I tried a video card that I know was working, and still nothing. What might the problem be, and how do I remedy it? Thanks- ( This is the DHP SFF )