Installed Radeon HD 4870 from 4850...computer not booting

Sounds like the PSU or something related to the PSU. Did you get the 20 or the 24 pin connecter plug into the mother board right? Have you tried the old PSU to see it you get the same problem? it isn't your 4870 if you are worried. What ever it is the power is bypassing the push on button on the front or top of the case. That is what makes me think that the one main connecter is not plug in right or it has a short in the wiring in side the PSU and putting power strait to the system.

I didn't do anything to my PSU, all I did was install the 4870, plugged in the 2 PCI-E power plugs (only using 1 card but it has 2 ports) and all this happened. I'll have to unplug everything and basically rebuild my old PC from scratch again.

BTW, I took out the video card and ram and it still does the powering on thing without those components in there.

Hope my PSU isn't fried.

The first rule of upgrading:
Change one part at a time if possible and test.

Second rule:
If it doesn't work, stop and back up.

Your post wasn't clear about if you reinstalled the old video card. If you didn't, remove the 4870 and reinstall the 4850. You need to go back to the original configuration to be certain that the rest of the system is still working. If it is, you need to test the 4870 in another working system.

A 4870 needs only about 30 watts more than a 4850, so power should not be a problem.

And once you have everything working, if your motherboard supports Crossfire and PSU capacity permits it, you can XF a 4850 and a 4770.

Ok, I removed every component until I got down to the mobo, psu, and cpu. Still the same problem. Once I flip the switch on the PSU everything turns on and idles. I also re-seated everything and re-wired all the cables again,

Tried switching out with another PSU, same problem.

Went back to my original config, same problem.

This is obviously a hardware death of some sort, and I suspect the 4870 fried something because when I plugged it in, everything turned on like it is now, but everything ran like it was in overdrive...like it was getting too much power. Case fans were spinning ultra fast, 4870's fan was running in ultra fast immediately, and the power was going off and on in rapid succession. And no it is not on the 220v setting.

I didn't think it would be a problem to upgrade from a 4850 to a 4870 with a 750w psu, but I was wrong apparently.

Is the mobo dead now? All that happens now is the fans turn on when I flip on the psu...don't even have to press the power button.

I feel for you. I know the feeling. I have had PC deaths before. I don't know what else to tell you unless you try another PSU. But Then again you could mess that one up to. At this point I would take it in and have a tech look at it and have him tell you what is wrong. I am sorry this has happened to you. I wish you the best of luck with your system.