Framerate problem with GTX 285

Hello, I'm new to this forum

I have a problem when playing some games, like assassin's creed, fallout 3 and a few others...
Anyway, I think should be running them pretty smoothly, but that doesn't happen.
I've got:

Windows 7 64x
nForce 730i
Intel Core 2 Quad Q6600 @ 2.4GHz
2x2Gb Ram Corsair Xms 800 MHz
EVGA GeForce GTX 285 SSC 1GB

I'm running with a little lower clock on the video card, almost stock-like and I've disabled that hybrid sli with the motherboard's video card, so I'm using only the GTX 285.
I've tried some driver like the latest 197.45 and also a 187.66 that someone recommended.

But when I run Assassin's Creed (the first one, not Assassin's Creed II) on 1440x900 60Hz, All maxed out, I get an average of like 25-30 fps. And I see on forums people saying that with specs similar to mine they get a smooth 60 fps.
Plus sometimes I get error that the some dll of the driver has crashed and the game just stops working.
Some games run smooth like Prototype and Dragon Age: Origins, so I don't know why I can't play Assassins Creed maxed out with at least 45-55 fps always.
I do get good fps if I lower the graphic settings, but a gtx 285 should run it at max well, right?

If anyone had any suggestions I would be glad to hear