Okay here is my problem. Before getting the 5870 I had a 7300GS and the computer also didn't have display not even BIOS or Post. The thing is that when I had the 7300GS it had the 1 long beep and 2 short beeps which means the graphics card is faulty says MSI in one of their manuals. I went and bought a 5870 and I now I don't get those beeps but I still don't have any display when I try boot. I'm not sure what else to do because I have no error beeps and my other components aren't faulty. Is this a motherboard compatibility issue or a power supply issue? I don't think it boots up properly at all as well. When I used to turn it on (when it worked) I used to be able to press the power button again any time before it hit the "Loading Windows" logo and it shut down. Now it doesn't even shut down at all when I do that.
Have you tried clearing the BIOS and trying again? It looks like the 5870 requires extra power to run. Are you plugging it into the PSU? When it fails to boot can you see or hear the hard drive spinning? I think the default for motherboards is one press of the power button starts it and then once it is running pressing-and-holding the power button for 4 seconds turns it off. If you hold the power button for 4 seconds will it turn off?