Matt_H mate HighGFX requires 2.1 and above as it's readme declares so I guess if you're using 2.1+ you're good to go.
You putted both HighGFX and it's info file in Devs:Monitors?
Also you can use Ratte's awesome SuperPlus Driver (800x600) after installing Indivision's latest beta core (both files exist on a thread over a1k.org).
I made a quick review over my Blog which you can check here (it has the link to the thread as well)
http://mfilos.blogspot.com/2012/01/a600-trying-800x600-resolution-setting.html