Well, RAM is very cheap now so I would go 4 GB. It will be enough for both XP and Vista for what you are doing.
I am guessing your motherboard probably will not support 4 GB, or the newer processors very well even though they are LGA775. Your best bet is to get a new motherboard that will allow you use a newer processor and use more RAM. Since you don't seem that interested in gaming look for a board that has Intel x4500HD (Something like that) integrated graphics. You could probably get a good combo deal for motherboard and processor off newegg.com