CPU improvements, in general, let you squeeze only a few more FPS out of the game. The jump from non-SSE to SSE2 might be around 10 FPS at most.
What really matters is your graphics card. Changing your OS just to have a few FPS more may not be worth the effort.
WiH intro with a:
Phenom II 970BE @ stock, ATI 5570 @ stock: 30fps
Phenom II 970BE @ 4.0, ATI 5570 @ stock: 35fps
Phenom II 970BE @ stock, ATI 5770 @ stock: 30fps
Phenom II 970BE @ 4.0, ATI 5770 @ stock: 35fps
Phenom II 970BE @ 4.0, ATI 5770 @ 950/1435: 35fps
All tested when the karuna gets gutted, and with the same nightly build, with everything I could find turned on and at max, yes including FXAA and Post Processing.
No, the GPU does -not- effect FSO untill you get into shadows (at least on a AMD comp), in which case both upgrading to a 5770 and overclocking said 5770 both showed major improvements. I was astounded to see a .5 OC bump the fps and my new 5770 to not effect them at all. Also, that 5570 is capable of running MVPs at 120fps solid (how DO I unlock it further anyway?)
Also, back with BPmulti Bigchunk and I did a 'how bad do apoco missles hurt fps' test, his 5770 was giving him the same fps my 5570 was at the time.
EDIT: I've been called nuts for thinking this has any preformance increase what-so-ever, but it is an AMD compiled EXE.
Have fun and please tell me your results in some way so I know if I'm insane or not.