If it's a Wallstreet, then 10.2.x is "technically" the last version of OS X installable on there. Panther "requires" a USB port, which a Wallstreet doesn't have.
However, with XpostFacto, I suspect you could install Panther on it.
No matter what you do, you should Max out your RAM if you're going to install OS X, but you might want to wait and max it out AFTER you get OS X installed. Some of those older boxes are notoriously picky about the isntallation processes when there's more than say 128MB of RAM in there.