What exactly are you considering upgrading: your OS, Logic, or both?
It sounds like you have the same iMac that I have, in which case you could move up to Logic Pro X IF you upgrade your OS to Mountain Lion first. LPX requires 10.8.4 or later. If you're thinking of Logic Pro 9, you can run that on 10.6.8 . . . you'd just have to shop around to find Logic Studio somewhere.
If you're talking about just upgrading your OS and keeping LE9, you can definitely move up to Lion or Mountain Lion. LE9 will run perfectly with either one. I would NOT suggest Mavericks, as, at least with the original release, you will likely have issues trying to use LE9.