Things to keep in mind
1: Old Rosetta PPC based programs will NOT work in Lion.
2: A machine using PowerPC processors can't upgrade to Snow Leopard or Lion, 10.5 is it.
3: A machine with Core Duo (not Core 2 Duo) processors cannot run Lion.
4: Lion is a RADICAL change, to the UI and everything, Snow Leopard is very less traumatic and offers performance tweaks in the video drivers over Leopard, as well as strips out the PPC code your not using on your Intel Mac's.
5: A machine as old to run 10.5 likely has old software, all that will likely need to be replaced under Lion, but perhaps not so much under Snow Leopard.
6: Lion is somewhat unstable, the 2011 Mac's with EFI 2.2 Firmware are being truckloaded into Apple Repair for new logicboards.
7: Lion has issues on older hardware that likely be last on the list for repairs as they are gathering information now about it's problems.
My advice, perhaps upgrade to Snow Leopard and leave Lion alone for a 2011 Mac or beyond hardware when you replace it.
Lion has got at least another YEAR of repairs ahead of it before I consider it as stable as Snow Leopard.
Everything from the AppStore, EFI firmware changes, Thunderbolt fixes and well as UI fixes, MAJOR security issues I can't disclose as well as all the third party programs requiring their fixes and updates.
Leave Lion alone for the time being, unless your a masochist.
And here's one bad security feature, anyone can hack your Mac with Lion like this.
1: Boot the Mac holding Command R keys and select the Terminal
2: Type resetpassword and press enter.
That's it. No disk needed.