HD 5850 will give you twice the performance of GTS 250 but definitely upgrade your monitor too.(For the card to show it's potential)
About BO,well this game has optimization problems.I've seen lots of people experiencing low FPS/Graphic glitch with the game.Some CFG tweaks and new patches will make the game run smoother though.
You can turn off PhysX,because first of all,not all games support it,second, it doesn't make a huge difference in graphics in most titles that support it.
I guess currently you play at a resolution like 1680x1050 or lower,if so,then a GTS 250 can handle all games fine at this res.That's why I recommended getting a full HD monitor w/5850 because using a 5850 on low resolution is a waste IMO.It really shines at high resolutions like 1080p