I have run Xp since it came out with Windows 2003 business edition
I had two 100Gb hard drives internally and eventually filled up the work one.
Now gone for a 1Tb hard drive. It came with Vista installled but I reinstaled Xp. I was worried a number of my old programmes might not like the cahnge and I would be unfamiliar with it.
I have now upgraded most programmes anyway to latest vista compatible programmmes and I have had a couple months getting used to Windows 2007 Office suite. Should I take the plunge, yes or no and with resasons if you can.
MickeySoft will eventually stop support for XP. Until that time, I would stick with what you've got.
MickeySoft have a habit of using the public to 'beta-test' their new products. Even if you WANT to switch, I'd leave it for a while to get more stable and more support from other manufacturers (i.e. drivers for hardware)
If XP meets your needs then stick with it until Vista has been out for a while longer and the major bugs fixed. If you opt for Vista you may have problems with drivers for some old harware, also some programs may not run properly [although they can be run in "Administrator mode"]
Vista is too Mickey Mouse & toy-town.
Personally, there is only one decent operating system: Linux.