Good First Person Shooter

[elwaylite]

I have had a PS3 since summer, mainly purchased it for media and BD's.

Looking to get my first game. I'm going to get Gran Turismo regardless, but Im a big first person shooter fan/splinter cell fan.

Whats a good game out now with good graphics thats challenging, but not too hard as to where when I pop it in once a week Ill forget how to play. With work, I dont have a lot of time but I'd still like to have on to pop in every now and then.

Also, anyone own the new Nascar? I've always liked these in the past on PC and PS2.

I was thinking:

Call of Duty 4
Or
Assassins Creed

Any more input?

Thanks guys.