(Haven't updated your profile lately have you? 10.5?)
Well that's what _I_ would have done also.
There really is no reason other then taking up some space not to keep the Leopard install. I'm a fatalist I guess. If anything you'll have something to fall back on if something goes wrong with the Lion disk. And you can still use the old programs until you have them replaced. I've had 3 or 4 different OS versions on my old Sawtooth tower at one time, just couldn't seem to part with them, well that, and I used them as repair disks for the others. (which you don't much need now with the "Repair Partition" in Lion). There was always one game the kids needed or some app that wasn't compatible with the newer OS that I had to use. As the disks died I just never reinstalled the older OS's.
Definitely clean install Lion and I imagine you have the Time Machine backup in bay 4?
...I have ordered a 3TB WD Green Caviar hard drive and plan to transfer my existing pictures to it. I am then thinking about re-formatting the 1TB drive, installing Lion and then restoring what applications I want from my Time Machine backup. I feel this will be a "cleaner" way to upgrade to Lion as well as a good way to clean house. I know that the PC programs won't work with Lion and I'm honestly not sure if I have any that I use/need but if I do I could keep them on the existing drive and (perhaps) have a second drive that could boot from and use Leopard if I need it? Regardless of that possibility, my plan would be to keep the current start-up disk as a "worst case scenario backup" ... at least for a while.
Thoughts and comments appreciated.
While a clean install offers more assurance of a good install as opposed to an upgrade, you're after not only a Lion boot disk but your current account on it, with all your applications and files (except the PPC applications that needed Rosetta). I've tried installing Lion a number of ways (an advantage of a Mac Pro with lots of disk bays and lots of partitions ) and the methods turn out to have trade-offs.
Using a clean install and then a Time Machine backup works but I found that software that requires registration and/or authorization often required it again and printer drivers tended to get messed up. Moreover, your using Time Machine to selectively restore "what applications I want" means having to also locate all of those applications' associated files sprinkled throughout the User and System Libraries.
Keeping your Snow Leopard drive, fresh installing Lion on the 1TB drive you wiped clean, and then using Migration Assistant should avoid more of the registration and printer issues.
I found the most successful transfer of "my stuff" to Lion was to start with a clone of my Snow Leopard installation on a clean drive or partition, and then using the Lion installer to upgrade that in place.
Finally, while you'll find a lot of disagreement on this, I think Lion is an acquired taste, especially for those who aren't coming from an iPhone or iPad to the Mac. Keeping Snow Leopard in place not only gives you a boot disk for the worst case scenario, but an OS you can go back to if Lion doesn't hit the spot.
I wanted to follow-up because I think it is helpful.
My experience installing Lion could not have been smoother. The transfer of data from the old disk to the new -- a process I was certain would be tedious -- was a breeze. So far Lion has behaved very well and since I do use iPad and iPhone a lot I think it will be very good to have all devices on the same OS.
I have browsed some of the Lion-bashing discussions and was a little nervous, especially since my Mac Pro is starting to age a bit. Learning anything new has it's moments but this has been one of the smoothest upgrades I've done. And it feels good to have the program on a clean hard drive.
Thanks again for the help here.