I have a problem just like yours... but my 9550 256 megs (mine is Connect 3D) worked fine on a K600 Chipset board... now that I moved to an K8T800 and AMD64 it just won't run.
on normal Windows XP I had your same problem.. I've gone a little further on XP x64 Edition but still mayor problems.
I've been working as a computer technician for over 12 years and 3D accelerated video cards since the First 3Dfx Voodoo Card, so I know what I'm doing, tested all possible settings on mobo and video drivers (and many drivers) with no happy results.
Now I'm moving to flashing the Video Card bios...
I really can't find the problem and everything goes nice on Linux (Fedora Core 4 x64) with ATI Linux Drivers.