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Vista does not support dual booting but you can have as many different operating systems as you want on your pc, eg vista and xp and ubuntu all at the same time.
You will need to use a boot loader program such as lilo or grub to load which ever os you want to run.
If you are planning to buy a new pc then then go to the makers support site for that model to see if they offer drivers for the different os, otherwise you might have reduced functionality in each of the os, eg the built in webcam might work in vista but not xp.
With regard to brand buy one from a big name- it will be more likely that you will be able to get support for it in 5 years.
Configuration- hard to say- how much do you want to spend? what will you use it for? do not buy anything that has 2gb of max ram or less. Vista is very resource hungry.