Not sure how much this will help TBC, but I set my machine as close to your settings as I could and here are the results:



This was done on the system in my sig using a 4870/512 OC'd to 790/1090...
Your score does seem a little low for a 5870.

Using CPU-Z check to make sure your PCI-e Link Width is set to 16x (circled in SS).

I can't imagine it making a huge difference, but it's worth looking into.
If it's set at 8x you'll have to find and set it in BIOS.

Hope this helps...... Dave