MS orginally intended to stop making XP available to Retail & OEMs after Jan 2008, but have have now extended this to June. System Builder License Availability end date is Jan 2009. Licenses will continue to be available through downgrade rights available in Volume Licensing programs after end of general availability.
Mainstream XP support will continue for 2 years after that, while extended (paid for) support will continue for a further 3 years (i.e. up to 2013). XP SP3 (which backports several of the better parts of Vista) will be available in January.
So your new system will be EOL long before XP is dead and gone.
My advice? Go for XP. Even on a high-end machine Vista is slower than XP most of the time. And it'll be a long time before peripheral makers stop including XP drivers.
Mac
Oh and seacue - "My life is based on DOS programs which run fine under XP. I hear that they will not run under Vista, especially the ones with graphics. Is that true?" - they won't if they try and talk directly to the hardware.
Either DosEMU or DosBox should run most of them fine under Linux.