Looking at the tests on
www.slrgear.com the newer IS lens - wide open out performs the older lens even when its stopped down two stops ... and by all accounts is one of the best kit lenses out there - keep it
Again looking at the tests the 18-200, while having a centre sweet spot, its a bit soft at the edges and not a match in the 18-55 range for your current lens.
As Alex asks - what are you looking for that your current set up doesn't give you?