It's funny you should mention Assassin's Creed. I just bought it yesterday via Steam, DL'd overnight and started playing this evening. So far, it runs great on my rig with all graphics options maxed out, 1280x1024. The only concession I've had to make is to lower the Crowd Density after I noticed the slightest bit of stutter when the crowds got too massive. Tweaking the Crowd Density fixed it. You might want to give that a try if you haven't already, under Gameplay Options.
Of course, I'm only a little ways into the game (just got to Damascus) so you might be talking about later parts getting choppy. We'll see. But so far, score another one for the x2!
Edit: Check out these graphs comparing your video card to mine (HD3870 vs 8800GT). That probably accounts for the difference right there -- the 8800GT gets about 15 FPS more in Crysis @ 1280x1024. Almost a 40% increase. Possibly Assassin's Creed is also heavily GPU-dependent like Crysis?