A 20" laptop is NOT portable. Sure you can lug it around but it's not a LAPtop anymore. I would much rather get an iMac or just an external monitor for a 15" or 17" laptop. I don't even like 17" laptops. They're just too big for me. I think 15" is the perfect size for a laptop.
Heh, it reminds me of the old days, where they used the term luggable (not laptop/portable/etc).