From my perspective, I know of 2 schools of thought. The first thing is to understand that XP will only see 3.5gb anyway, so you can put 16gb in it and everything over 3.5 is just wasted. Then move on to your motherboard. How many slots?

The schools of thought come in here. A lot of people want to buy the 4gb stick and have one stick in there. If your mobo has 4 slots, I believe in 1gb sticks time 4. Then one can go bad and you aren't dead while you wait for RAM to come in the mail. OR 2-1-1, etc.... whatever your mobo form factor is.

Of course as cheap as RAM is now, nothing says you can't use a 4gb and have a spare 4gb in the drawer.

I can't even guess how many people first bragged about having 16gb of RAM in their XP machine and then argued when I told them it only accessed 3.5gb. Yes, the computer will SEE all 16gb in BIOS (BIOS doesn't know what OS you run), but the OS will never write to it. Learned that a long time ago from Microsoft themselves when I wrote nasty letters asking why they limited the RAM access. I think the exchange with the tech was up into the 20s. Back and forth we went.

And I am sure someone here will fire back about how their computer has a semi truckful of RAM in their XP machine and it "uses" all of it. My response to that has always been something close to "Believe whatever you want to believe. My life doesn't change if you are wrong." Maybe they have that one "special" version of XP that uses more RAM.....