they are basically the same board.
the default clocks are 450 core, and 800 memory for both 8500gt , 8400gs.
xfx though has a lot of models, but the 450/983 shader and 800 memory is the nvidia standard (xfx has some with 667 and 600 and 800 memory)
you'll notice that this particular one has 450 /1200 shader and like 600 memory. which is pretty bad since the memory speed is most important.
i'd say its probably slightly slower than a normal 8500gt with those clocks.