until your are not happy with windows xp i would wait at least 1 year until you upgrade.

normally i a product life circle you will run into vista anyway.
for example you are using your computer for 3 years and after that you buy a new one. the chance that you are planning to buy a computer next year might be high. in this case just buy it with pre-installed vista.

never change a runnin system. its 50/50 that you might get in trouble if you install vista, wait for service pack 1-2 .

and i think xp is runnin quite good .

dp