CatalystPM 4.20_m257UB_HD was the last public release that will work on a G5
samsc.com/showthread.php?t=1662

I don't think m255 was a 'public release' - unless there are specific custom things you use - then m257 is probably the best stable version for a G5.
(there were a few interim private releases too - contact Richard)

Richard changed the code in CatalystPM 4.40_m329_HD and it only works on intel MacPros running 10.5.8 or 10.6.x - not G5's
samsc.com/showthread.php?t=1907

S