Hi Weasel. As with every other iteration of Windows the software developers write software designed to take advantage of the features of the new OS. You end up getting fewer and fewer programs that will be able to run well on the older OS.
Also, Vista is a 64bit OS, XP is a 32 bit OS. New software will be made to run on a 64bit OS and also take advantage of the features of the newer Direct X. A 32bit program will run on Vista but a 64bit program probably won't run on a 32bit OS unless it is made to do so. I don't thiink many software developers will go to the trouble of doing that. At least that's my take on the whole thing.
Bottom line, you will eventually be painted into a corner and will have to buy the OS. That is how it has worked in the past and, unfortunately, that's how it will work in the future. Microsoft is the Event Promoter, the software developers are the events and we are the spectators.