Many 550 Ti models are close to the same price as Radeon HD 6850's, while also being slower. I'd go with a 6850 instead. Witcher 2 eats enough GPU power that an HD 6870 wouldn't be going to waste, either.
I doubt you'd be happy with your performance/visuals in that game, at that resolution, with a 550 Ti. The GTS 320 shouldn't even be on your radar, that's comparable to GPU's of 5+ years ago.