Hi Joltz,
I had a similar problem when installing a Sapphire 9800 Pro (128mb) recently. It drove me mad, im assuming that after installing the card the first time, it booted up okay (before any drivers were installed)??
I found that the problem was that 1) My existing drivers still existed, and 2) The drivers/software that came with the card knackered my system. I uninstalled the drivers and used the Cat 5.4 based Omega Drivers instead (almost the same, but just the basic drivers - they work perfectly!).
Here is what i did, may be it will help you.
1) Uninstall ALL drivers for your system (old and new). Do this by downloading the following program, run it and uninstall any drivers from both your old AND new card:
http://www.drivercleaner.net/dc.htm
2) Download and install the Radeon 9800 Pro OMEGA DRIVERS, they will run your card just fine. These drivers can be found at:
http://www.omegadrivers.net/
Best of luck, hope this helps.
James