rjspring: If you were interested in a full fledged laptop, the Dell outlet also has some great deals (may come close to the cost of a netbook) and would probably have the processing power needed to run the GOG titles.
The graphics chip is going to be the main thing to consider, since there are no GOG's that will be limited by either a Core or Athlon processor. The GMA 4500mhd is pretty decent, being about the same speed as the old Radeon 9600. Any discrete (not integrated) ATI or NVidia card will blow the GMA chips out of the water though.
You might check TigerDirect and MWave. Sometimes they have pretty good deals on full size laptops. Though anything with a GMA 4500 or better is probably in the $500 range and up.
EDIT: Here's a list of all the mobile graphics chips and their relative performance, ordered from best to worst. If you can get something in the upper part of Class 3, or higher , it should play every game on GOG.
http://www.notebookcheck.net/Comparison-of-Graphic-Cards.130.0.html