I think anything past a 9800GTX (GTS250/450) dedicated for PhysX you won't see any gains, such as right now I'm using a GTX260 216sp for PhysX alongside my Tri-Sli GTX480's and swapping it for my HTPC GTS450 there is no difference in physx games.
The 8800GTX is old tech, not power efficient and is dual slot. The 545 would be more than capable for your needs.
for borderlands, did you have some sort of benchmark for that? or were you just going in and running around shooting stuff?
if you were just running around, then these numbers are ok as a baseline, but i wouldn't take them as gospel. if the tests weren't made with the exact same scenarios, same shooting, same enemies, the numbers aren't going to mean too much compared to each other.