Hi guys,
I've seen there are a few threads on this topic but I couldn't see anything that applied to me. I've just bought a new pretty much everything for my PC. The only original part now is the case and PSU. I had problems with the hard drive of the original unit and wanted some more RAM and an updated graphics card so got myself a gig of ram, a new HD and a Radeon PowerColor 9550 graphics card. I put this all into my old machine and got a no signal message from my monitor.
I assumed there must be a problem with something else as the card was new etc so I've just got myself a new Mobo which is an nForce3-a939 and an AMD Athlon 64, 3200+ processor. These are now in and I'm getting the same problem with the no signal message on my monitor then it going onto standby. Everything in the computer seems to be booting up properly but I don't get any beeps ( though I can't see an onboard speaker on the mobo). I've tried it with an old graphics card and still no joy. The only old things now are the PSU and case as I said before. My friend thinks the problem may be with the case and that this is causing the graphics card to not connect properly but I can't see that myself. Is there anyway it could be a problem with the PSU although all the fans are spinning etc. Any ideas would be appreciated thanks.
I have tried with multiple different monitor and still no luck.

Can you hear the system running ? I assume you have any auxiliary voltage adapters connected to you graphics card. Do you have a voltmeter to check power supply output ?