It always takes them a while to iron out the bugs, so I would suggest not upgrading to Lion until they've issued an update ot two ...

I'm still on, and perfectly happy with, Tiger on a PowerMac (i.e. non-Intel) that runs Poser 7 and Pro 2010. I do have Leopard on a second drive, but that's just for a couple of apps that don't like Tiger.

And I've had Photoshop since version 4, and Mac since System 7.5.3 and the various upgrades have always played nicely for me. My old machine has Photoshop 6 and OS 8.6, still working happily (though the G5 is a lot faster ...).