WITHCHER 2 poor performance and rendering

I had an 8800gt and it burned out -replaced it with a palit gtx560ti and its simply pathetic in comparison.
Graphics drop out - no rendering in places and never more than 2 seconds without jerky play.
New drivers - uptodate patches - clean install nothing has made any difference - is this just a poor card?
The Gainward 800 played smoothly and looked good enough for its age. An upgrade shouldnt be a backward step - especially at these prices....
and No I wont upgrade to windows 7 just on the offchance it will improve the performance of my card!

Even playing on minimum the results were tragic.

(palit gtx 560ti - amd 3000 dual core 4gb ram vista)