Max amount of RAM will and always will be on the motherboard specs, most motherboards support upto 4gb of RAM and modern mother boards will support upto 8GB. The other thing you'll want to make sure is that the new RAM goes in the same channels as each other so as to get the dual channel speeds of them.

Finally if you have a 32bit OS (Anything pre-XP+XP and some flavours of Vista) then don't bother going over 3 gig of RAM as Windows just won't pick it up.