If you have no need to run PPC apps, then you can proceed. I suggest making a clone of your current installation onto an ext Firewire HD and installing Lion/Mtn LIon onto it for testing purposes. BTW, you should probably make two clones. One for testing and one to restore your current state if it goes south.
It is indeed all down to what you need to do with your machine. Snow leopard is an excellent OS and as Baltwo suggests you have to check out your current apps and programs because PPC won't work on any higher OS than the one you have.
Andy's comment is very interesting and, I suspect, insider knowledge based Certainly I would wait for Mavericks now it is on the horizon ... and then add six months or so for the bugs to be ironed out.
Good luck, whatever you do. I'm staying with Snow Leopard.