I never had a problem to begin with. But you should probably check 5.6 first because 5.5 was where ATI did a whole bunch of revamping to the driver and 5.6 is more or less a bugfix. I've used Omega before, but they just aren't updated quick enough IMO.
Try ones based on 5.6, if it works weird (esp. 3DS mark 2k5 and HL2), try omegas based on 5.1.
Cats were always bugged here, even after clean install, some versions more than others.
Any of the Omegas give better results (but may be slightly slower on some games, not by much though,
if you play mostly PSX, don't even hesitate)... in my case that's prolly cause my 9800peo is 128bit, which isn't really taken seriously when Ati makes drivers