These answers are already all over the place. There are several topics on the whole RAM and Vista/XP issue.
XP will only be able to use 3GB. Havign 6GB wont do you any good with a 32 Bit system. 3GB would be wated.
64 Bit would be able to use the 6GB of RAM. But honestly it is overkill.
As for the whole XP/Vista thing just look over the first couple of pages here. You will see several topics already about this. We can not answer this for you. Only you can answer it. Do you prefer XP? Do you like XP's speed? Do you like the eye candy of Vista? Can you handle working wth VIsta and it various quirks?