DX10 adds a number of particle, water, and smoke effects, as well as slightly nicer shadows. It is not worth the money to move up if all you're looking for is eye candy.
Assuming you have the hardware to run Vista right, however, running a DX10 game, using Vista and DX10, but turning off all of the enhancements (essentially making the game run DX9 graphics in a DX10 environment) WILL yield slightly better framerates. Not enough to make it worthwhile, but a little. This is the result of performance optimizations present in Vista.
Right now I'm thinking about upgrading to 64-bit Vista and getting another 2GB of memory. But something like that would probably be a couple hundred bucks by the time everything is said and done, and the performance improvements won't warrant that kind of a cash outlay. But it would look a little prettier.