Just to let you know
1: Lion needs 2GB, but prefers 4GB of RAM
2: Requires a Intel Core 2 Duo or more recent Intel processor
3: No Rosetta, a lot of your 10.5 / 10.6 programs and drivers may not work
4: Your older older machine may not have the ability for trackpad gestures etc.
5: If your machine isn't a Early 2011 Mac or later, your results with Lion may vary, you might have a lot of issues others report on these forums with older hardware.
6: You can't use TimeMachine to restore to Snow Leopard once it's been hooked to Lion, only by making a Snow Leopard clone on a external drive. You'll need to COMPLETELY erase the Lion drive hold c booted from the 10.6 disks before restoring a Snow Leopard TM or clone, this is to erase the hidden Lion Recovery partition installed on the drive.
7: You should update all your third party software for Lion, especially any that install Kext files, or remove these third party software and their kext files from your System/Library/Extensions folder while your still in Snow Leopard.
This Terminal command will list the non-Apple kext files (doesn't work while booted into Safe mode, aka hold shift key boot)
kextstat -kl | awk ' !/apple/ { print $6 } '
copy and paste the above command, do not type it, and press return to list.
8: If you have a issue and Lion is stuck on a spinning grey screen wheel, reboot holding the shift key, this will disable the QuartzExtreme and third party kext files.
9: Lion is slower than Snow Leopard, for maximum performance on Lion, backup files, erase and fresh install 10.6 then upgrade to 10.6.8 then to Lion and update, then install Lion ONLY programs from fresh sources and finally files from backup.
10: Maintain two separate hardware copies of your data in easy to access ability (not all TimeMachine) in case you need to rebuild the operating system from scratch or if your TM drive is borked and you don't know it yet.
https://discussions.apple.com/thread/3358920
11: Lion changes a lot of things, your going to lose Front Row, Spaces, Exposé, the scrolling direction is backwards and more restrictions and noob proofing. It might make you angry, it might not work, however if you clone or keep your TM drive disconnected, you can recover to Snow Leopard without much fuss and leave Lion for a new machine as Lion begins a whole new level of bloating to retire older hardware prematurely, likely for quads cores and up.
Good Luck 🙂
