I've installed Fallout 3 New Vegas and I can run it on pretty high settings just fine.

The problem I've got is that whenever I enable Antianolizing the NPCs stutter horribly when they walk/run. I wouldn't call it 'lag', it's more like they are just flickering/stuttering when they move.

Has anyone else had this problem? If so did you find a way to fix it?

I'm using an ATI Radeon HD 3870

Anti-Aliasing can be a real performance killer and that card of yours isn't the greatest. I'd say it simply can't handle having AA turned on, unless you drop some of your other settings.