There are OS optimizations for the G5 too.
[link|http://www.apple.com.au/powermac/|PowerMac G5]:
Specially tuned for the G5 processor, Panther provides a seamless transition to 64-bit power with optimized libraries that let today\ufffds applications take advantage of tomorrow\ufffds power.
I think the G5 optimizations are supposed to be more extensive with Tiger (10.4).
I don't think that the future OS capabilities should be a concern though, not for an inexpensive, lightly-pushed desktop anyway.
Cheers,
Scott.