I remember when Oblivion came out, my pc i had at the time couldn't handle it and had to run it on low low low. Got it again on the 360 and was much improved but i still prefer the pc version. Will have a look at the next iMac refresh to see what kind of graphics they are sporting. Ideally id like the 21" with improved graphics as the 27 is too big for me.
I actually think the system requirements will be fairly low. I'm sure most computers (even if they are a bit older) will be able to run it. You have to keep in mind that the developers want the PC / PS3 / XBOX 360 version to look similar. And seeing how the hardware for both PS3 and XBOX 360 hasn't changed and is still the same as the hardware they had to utilize for Oblivion, they probably will keep the system requirements for the PC on par with what an XBOX or PS3 is able to do... if that makes any sense.
Of course the PC version will be the better looking one, and require a bit more horsepower. But I really think we'll be able to play this game on fairly low settings... (or at least I hope so
-Brun