A good NOTEBOOK PC would use a Pentium-M processor for long battery life, would sport a 15" screen, and would weigh under 7 lbs. The Dell Inspiron 6000
is a good example of such a notebook for portable computing with the performance of a desktop PC.
However, you made it clear that you aren't looking for a notebook, but a LUGGABLE desktop replacement PC. In that case, you can spend less, since the Pentium-M is priced rather higher than other X86 processors.
A luggable with a 17" screen and an Athlon64 might weigh 9 lbs or more. Some of these have excellent graphics processors that even make them suitable for gaming. A cheap Pentium4 system might get close to 12 lbs, due to the need for heavier batteries and less use of weight-saving components, but these are the most affordable of all luggable desktop replacement PCs. Pentium4 is a terrible processor for a notebook because it consumes so much more power and dissipates so much more heat than other X86 processors, but if you want a cheap desktop replacement, this may meet your needs for the least amount of money. If it's sitting on your desk, your lap won't be burned by the heat and the wall socket can provide plenty of power.