I don't see how taxing Leopard can be. I've got a lime iMac DV 400 here running Tiger just fine (yea sure it's on the slow side but it's running~). That's a 6-7 year old computer running the latest Mac OS. How many 6-7 year old PCs can usably run XP? How many people even use 6-7 year old PCs as their main computer? (re. the longivity/resale value of Macs)
I don't think Leopard will support this old horse considering most of Apple's software now requires a minimum of G3/400. Looking at the difference between Tiger and Leopard, and XP and "Longhorn" (sorry, can't resist

), though, I doubt Leopard requires much to run. I'm pretty sure anything above 600MHz can run it.
On the other hand, there was an article somewhere about the MB and MBP's ability to run Vista. From what I remember, only the higher end MBPs can run Vista fully (with Aero and all), and even then it requires at least 1GB of ram. One word: Bloat.