refer to 32bit and 64bit versions of Vista. Up until fairly recently all computers operated as 32bit machines, but you can now buy 64 bit processors, and Microsoft has made a 64bit version of Vista for these new processors.
In very simple terms a programme running on a 32bit machine can address up to 4gigabytes of memory. That seems a lot, and it is more than enough for the average home computer user. When you contemplate using a computer for working with huge datasets - the kind of thing you can get with scientific research, or complex autocad projects for instance, you begin to realise that a 32 bit machine can struggle, and that's where 64bit computing comes into its own. There's virtually no limit to the memory that can be addressed (well there is, but it's so huge it's not worth bothering about.
In short, 64bit computing is (at the moment) not something that the average computer user should consider at all essential, despite what some people would have you believe.
Other than that, your choice of Vista version depends on your personal computing habits, and on the hardware configuration of your machine. You should download and run the Vista analysis tool from the Vista website; it will tell you if your computer is suitable. Then it's just a matter of making your choice - the ultimate version is only something you should consider if you particularly like its multi-media and encryption facilities and , otherwise it's no different from the less expensive versions.