I'm running Vista Home Premium x64 on an Asus P6T Deluxe v2 with 12 GB DDR3 and an Intel Core i7 920 and two crossfired Sapphire ATI Radeon HD4890 OC's and the sim runs like butter and it screams on 1920x1200 with full detail and the Fillrate Optimiser on 100%.
I've tried the slider to the left and it sucked major golfballs...and I tried it to the right and I saw little improvement and a slight decrease in fps' in AI intense close city combat.
My advise is not to touch the slider. Yet it is fun fiddling around with it when you're a tweaky person. ;)