Yes, Vista allows for font-resizing on the GUI.
XP is no longer available as an installed OS although you can get a 'back-grade' to it from some manufacturers.
If you are MAC now, you will probably like the interface better in Vista. XP is a 7-year mature operating system and quite robust.
If Vista,
Processor, RAM, Graphics Card are all critical to Vista performance. 3GB of Ram is key to success. 2 will run, 4 will sing but 3 gets the job done. Just my opinions.