cleeve

Illustrious
If they have the same core and memory clock speeds, they will perform identically.

If one is clocked a bit higher, you can expect up to a 5% performance increase, but nothing you'd be able to see with the human eye.

The only difference would be if one has DDR2 memory and the other has GDDR3 memory. in that case, the GDDR3 verison would be notably faster, probably in the realm of 25%.