CountChocula
Keeper of the Watch
I run Oblivion and The Witcher 2 on high and get 40+ on fraps. It bogs a little if there are lots of creatures. I'm even running Qarl's highest end textures in Oblivion and doing fine. I guess I'll wait and see. Money isn't an object, but laziness scores high. Also, getting a power supply to fit inside the Dell case can be a pain. The GTS 240 supposedly requires a 450 watt PSU and it runs fine in the Dell. Of course, they could have tweaked the card so it uses less power. Decisions, decisions. If anyone is a power supply guru and has a guess as to what would fit in my Dell, I'm all ears.
It is remarkable that you can get similar frame rates for both Oblivion and Witcher 2. Perhaps Witcher 2 is more CPU intensive than I realized.
If you get 40+ fps for Witcher 2, on high, I would imagine you will be just fine with Skyrim on Ultra settings.
On my laptop with i7-820QM, AMD 5830m and 1920x1080 display, Oblivion runs at constant 60fps with vsync enabled and Fallout New Vegas runs at about 45-50 fps; however, Witcher 2 is a barely playable slideshow, even with all graphics options at the lowest possible settings, 20-25 fps, often dipping below 20 fps. It must be your relatively powerful desktop CPU that enables you to get such high frame rates for Witcher 2 at 1680x1050.
As for your PSU, you may want to double check on a forum like http://www.tomshardware.com/forum/ or http://www.overclockers.com/forums/
However, it sounds like you probably already have a 450W PSU running in your system.