beerdrinker,
If you get a Sony machine with Vista ultimate installed, you shouldn't have to pay for a "downgrade"
The licence for ultimate also covers an XP professional installation. and I believe that Vista business does as well, but I'm not 100% certain of this. (You need to get hold of a copy of an XP installation disc, but you don't need the microsoft key)
About 18 months ago, I purchased a Toshiba laptop with Vista Ultimate and hated it, so I did the downgrade. I borrowed an XP disk, installed it,and just entered the key from Vista for activation.
The only problem you might have is getting the drivers for your laptop. I had a hell of a job finding a display driver that was XP compatible, but Toshiba managed to help me out.