welll that really isnt much of an upgrade, the 9600xt is 3 years old.
I recon the 9600xt is on par with a mid range GF5 (5700 Ultra). A top range GF4 should spank it, so if that GF4 of yours is a top model I suggest you swap back :-/
Hmm, I think maybe the mx440 was running in DX8 but the 9600XT is capable of DX9 so its running that by default.
This would explain why your FPS has dropped.