Nope- don't plan to.
A bit of history- in 1995, I got a 486 100mhz (which was the fastest 486 at the time) ran it with Windows 3.1 until 2003 when, finaly, everything went goofy. I never upgraded to Windows 95, 98, ME or any other because all would take up too much space and make the thing run slower.
I now have a Sony Vaio I bought in 2003 with 1.2 ghz Celeron, 60 gig hard drive, a DVD drive and a CD-RW drive runing XP. It works great. My wife gave me Flight Simulator X for Christmas and a kickin aviator joystick.
Too graphics intensive for the machine, so a 128mb graphics card, another 256 mb RAM, and, 'cause it was cheap, an extra 120 gig hard drive, I've got something that works great. I just got a USB 2.0 upgrade card (to transfer video onto the iPod) so for about 150 bucks, I've got something that works great for me.
XP works fine, so I see no reason to overtax a system that works fine for the gee gaws I don't need.
I'm sure Vista is neat, but I don't see the need.