Again, the max memory for either system is 512 MB.
In order to have more, you'd need a new MotherBoard, in which case your current Installation of Windows would cease to work since it's directly-tied to a Compaq system (ie - MotherBoard). The only thing you could do would be to buy a new system - it's really not feasible price-wise to go through the hassle of getting a new Board, more memory, and buying a copy of Windows for a system that old, when a newer/faster/more upgradable system will run about the same price as the upgrade, or perhaps a bit cheaper once everything is factored into play.
I know it's not what you want to hear, but it is what it is. Sorry.