Audio and video editing requires as much ram as you can throw at it, otherwise you'll just end up swapping to the hard drive all the time and it will be slow.
Assuming you have a 32bit version of XP then it won't see anything over about 4GB (and will use around 3.5 of that)
So simple answer, the more the better upto 4GB, but you'll see a huge difference just going to 2 or 3GB.
(1GB is the bare minimum I'd suggest running XP on it's own with these days)