For the Max724; R1 (resistor #1 in the circuit diagram) for 5 volts output is 2.8K ohms. As per "knarf" changing R1 in the circuit to 3K ohms gets you 5.2V. I don't know it this will be the same for the MAX787 that you mention that you said you are using.
The CompGeeks adapter sounds like a great way to go if it can supply enough amps. My webplayer is modified with a hard drive and cdrom so I may need as much as 5 amps, I am not sure. Let us know if the adapter does OK and what additional loads (hard drive, fan, etc) you have added to your WP. Another thread quotes the plug at 5.5mm O.D., 2.5mm I.D. Radio Shack part 274-1573; I have not verified that info.
Let's keep this thread going as I want to get this power supply thing finalized to use my WP for MP3 player, mapping program, etc computer for the car on roadtrips.