Black Ops 2 itself isn't too demanding so the GTS 450 should handle that at high settings with no problem (tested with my GTX 550 Ti which is only marginally quicker at 1680x1050) since it uses the old COD engine anyway
In regards to most other games, medium-high settings at 720p is attainable, or low settings at 1080p
I do wonder why you're buying a rather old Fermi card, is this second hand? otherwise you could at the very least step up to a 7770