Troubleshooting Help

Hello, I'm new to the community, but not new to the site. I have alot of troubleshooting experience but am working on a computer and am i guess stumped. It is a dell dimension 8400 that I replaced the Power supply (dead) still wouldnt post so I ordered up a new cpu thinking that would be the problem, still wouldnt post. Tested and seems it would be a dead motherboard so i ordered a new one, when I put the new mobo in everything seems to turn on and fans and hard drive seem to fire up but still no video. So I replaced it with what i thought was a known good video card and still no video out. And this is on the new mobo. I have taken out all the ram sticks and tested a single one in all the slots and it makes no differance. Even when I take them all out no beep so I'm not sure if I have the pcspeaker connected..I am getting a solid orange light on front, and no diagnostic lights on back. I'm not sure what else to try.... Any help would be appreciated?