CPU depicts what OS you can run, in this case Intel has had 64bit chips since Pentium 4. No worries there.
Go to the motherboard manufacturer's site. Easiest way is to simply GOogle the model and it'll be the top site.
Did it for you. You'll have to figure out what GPU is in it.
You're limited to DDR2, and can get some cheap DDR2 off Newegg if it requires more. If this is for your parents and all they do is browse the net, do email or whatever then 4GB should be fine. Check periodically to see if they are capping RAM, add as necessary.
Why would he want to do that when it'd be cheaper and better to go Skylake?