Under windows 7 - 64bit i find that OMSI runs better if you use compatibility mode for Windows Vista and disable Visual effects in particular windows Aero.
Im going to try also if setting processor affinity (the ability to give 1 of your CPU cores to a .exe program) will have good or bad effects
For the record I can get Grundorf to touch 100 FPS on external camera with certain settings but it wont stay there, dropping to 50fps external and 30fps internal driving the Merc 0405N2
My setup which should be beaten by all of you with an average Core i3 and especially Core i5 / i7
AMD Athlon 6400+ Black Ed (Stock 3.2Ghz)
Asus M2N SLi Deluxe
3GB DDR 667 (PC5400)
500GB Western Digital SATA2
ATI Radeon X1650 pro (512mb)
750W PSU