Your Bios is Award Bios am i Right?. You say you get one beep. if this is a short beep?. On this Bios, it means it has passed the post is it a single short beep?.
But ok you still having problems, now try pulling out all the cards in the pc and see if you hear diffrent beeps, some can be short others long. Now put your Graphic only back in see if any message on screen come up or if somthing has changed, like you hear different beeps or see Different errors on screen. Replace the cards one by one rebooting each time looking for changes. If you dont have any look at doing this. Can you Find a diffrent Graphics card to try. Maybe borrow one of a fried just to try.
ONLY TRY THIS IF THE ABOVE DOESNT WORK
If you still have no luck after trying another Graphic Card, there is one last thing you can do to see if its the Motherboard, Ram, Cpu, Heatsink Fan. That is to disconnect everything from the back of your computer, except for the power lead. Now disconnect all your drives, Hard Drives DVD the lot. Now take out all your cards Graphics sound the lot. Now reboot the computer so there is only the Motherboard Ram And cpu heatink and fan. Do you hear any beeps how many short or long. Let me know how this has gone. Also if it works.