True, but consider product life.
Windows 95 lasted for 6 years.
Windows 98 lasted for 8 years (Supposed to be dumped in 2004, but they extended it another 2 years when Google reported about 40% of the hits it received were from 98)
ME for 6
2000 isn't due to be phased out until 2010 (Although they could extend that like they did with 98: that would make it supported for over 10 years!)
I imagine XP will be supported for at the very minimum, another 5 years, (I'd guess 7 or 8) Mostly because it's the most used at the moment. Vista will pass it in about 2 years, but it will be supported.
I say go with XP. There's no reason to go to Vista at this point in time.