heh. where is the 'mac' when you need it?!?!? a headless version of the iMac, but not quite as small/non upgradeable as the mac mini. would have a full size HD and a non-integrated video card (like the iMac). maybe even a PCI express slot for the video card (similar to the old CUBE, but could accept a full size video card) the 'mac' would be between the mac mini and the mac pro!!!!
until the day there is a 'mac' id ask yourself how long you want to keep the iMac. if its for a long time (like 4-5 years) then go for the 256mb in the 20 inch iMac, the 256 will be more 'future proof'. otherwise if you want to keep if for about 2 or 3 years then go for the 17" and youll survive with the 128mb of video.