Do a search. Almost all of these questions have been answered already.
Yes Vista is worth it.
There are few compatability issues. Most of them stem from upgrades since people dont remove the old applicaitons that are not compatable. Check http://windowsvista.com/
for the Upgrade Advisor. It will tell you everything.
The nVidia issue is being worked on. They are working on new drivers. It isnt fully sorted but they are working. Vista comes with DX10 installed. You can run the DX9 runtimes with no problem at all.