@ Tenacious
All I can come up with is that you check system's recommended requirements against your hardware. I think the most recent OS, the better as long as it runs fast.
But the biggest leap in Mac OS (except for 9 -> X transition) was between 10.3 and 10.4. Just check out the features comparison on the website. Mac OS 10.5 has some improvements, mostly UI smoothing, making it more Vista-like (thus making it more 10.4-like :-))