Perhaps you should stay with Win98 as the same people said the same thing about WinXP when it was introduced.

The main difficulties are that older software and hardware can misbehave in Vista. The best thing to do is to check if your existing hardware and software are compatible with Vista. You only need concern yourself with the peripheral hardware and the installed software, not the RAM, CPU and Graphics.
Plug in all your equipment then download and run the Vista Upgrade Advisor from here http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/upgradeadvisor.mspx

Compare the features of different versions of Vista here http://www.microsoft.com/windows/products/windowsvista/editions/choose.mspx

And, yes you can still buy PCs loaded with XP.