Did you install it in that Dell from your sig? You might try uninstalling the drivers for the Radeon, and the ones from Nvidia if you haven't already. Then boot into safe mode, use Driver Sweeper to clean out any left-overs from both Nvidia graphics and ATI graphics, reboot normally and install latest drivers for your new card.
Other games don't work either, or just SH5?
Porto; 5870 is the fastest single GPU card on the market. What more do you need to know about performance?
