honestly, throwing that resolution down won't do too much.
At least on my setup, when I had the resolution cranked up, I had a nice stutter-show,
and when I lowered it to the screenshot size here, it really got nice and smooth, so it has to do something. :-?
It would seem to me that the higher the resolution, the more detail and information the system has to process and draw on the screen.
And if the sims are not very GPU dependent, then that must mean that the CPU is taking part of that hit as well.
I know that frames aren't important, but I can keep a steady smooth 30fps on my system with all sliders to the right, and the OP's system easily tops mine.
So for me to have better frame rates and smoothness than he does, doesn't make sense.
And I have never done Nicks XP tweaks. Imagine what I could get if I did.

(Holding off for the fresh install that I know is coming soon :-/

)
Or I could be spouting utter madness!
