What is it you are lacking...
I recently upgraded out of a 733 to a P4... For gaming and some dscaler filters this made a difference if trying to do ffShow with TomsMoComp it was needed but I can honestly say it made not one once of difference to my DVD playback...
If you are trying to do something and your HTPC is too slow then focus on fixing it, if it is just the desire for more power (URG URG URG

) then what do you need it for... My 733 used to run Tom's GreedyHM no problems with dScaler and I was content... Free PC parts came my way so I built another box...
Arguably the 9xxx series of radeons and new catalysts perform better than the lesser (7500 more so than the 8500) series of cards but dpending on your display I would say a MP modified 7500 would kick a 9xxx anyway... Perhaps a 9xxx modified by MP is a good investment...
Really without knowing where your system is letting you down or what you want or need to do, its hard to advise you on what changes to make...