Strange lag drop

Hi guys,
i have a problem here.I can play every single module for Warband (well not FI2) on ultra with no big frame drops,but i can't play NW on dx9 at all.It lags even on lowest graphic.If i put it on dx7 i can play on ultra with 60fps,but on dx9 i can only get 20fps.
I have tryed to roll back my Nvidia drivers,it didn't help,change redender buffer size to 500,didnt helped,lowered my graphic on lowest,didnt helped.
My pc specs are :
2.8ghz Dual Core
2gb ram (i will buy more ram)
Geforce 8500gt (512mb)
My Nvidia drivers are latest,but roll backing didn't helped.

Is there any way to play nw on dx9 ?

Buying more ram memory won't improve the performance of your game at all. Actually, having more than 2 GB of ram memory is useless for running Warband.

Try limiting your FPS to 30 and see how it runs.

I buyed 1gb of ram.Actualy it helped,cuz my graphic card use some ram memory.Howewer it's still unplayable on dx9.I'l try to limit fps to 30 (it was limited to 60 btw).
--
Changed to 30fps limit,it lags even more.Damn't looks like that i must  play it on dx7.