I had a similar problem with a 7800GT and my son's computer. No matter what I did, it just wouldn't post. I switched PSUs, checked to see if anything was shorting. Different drivers, everything. Never got it to work in his. It worked perfectly in my machine though. So I ended up sticking that 7800GT in my comp and getting a 7900GS for his.
One thing that was suggested to me was to go into safe mode and set the resolution down low to something the monitor could easily handle. Like 800x600 or even 640x480 at 60Hz or so.