The next windows version after vista is touted as windows 7. however they could change its name like the vista/longhorn palava .... Its not due for a couple of years at least, however with vista being compared to winME it could be sooner ....

I would suggest winXP. Your going to suffer less compatibility problems as well as more performance for your hardware .... Ots supposed to be supported with updates till 2014 and its still one of the best windows versions, maybe win2k aside ......