PC (obviously). With mostly graphics mods (Oblivion Mod Manager, Qarl's texture upgrades, DarNified UI, Illuminaton Within, et al). I have a vaguely studly rig, so Oblivion doesn't faze it.
I played through Knights of the Nine, but Shivering Isles looks like one of Tim Burton's more disturbing dreams, and I haven't been very interested in finishing it.
Just recently heard about a large mod called Nehrim [http://www.nehrim.de/startEV.html], which looks interesting. Problem: I'll have to brush up on my high-school German in order to play it.