Here is a list of all the marketplace content... click a title to see the size
http://www.achieve360points.com/live/ 
according to that site the average game demo is 450MB and the average XBLA game is 25MB
Assuming the full bandwidth of 56K it would take you ~18.25 hours to snatch an average demo and as much as 92 hours to snatch the largest game demo, it would take you ~1.1 hours to snatch an average XBLA game and as much as 2 hours to snatch the largest XBLA game.
So XBLA games are do-able but I wouldn't bother with game demos... just order a subscription to OXM.
This post has been edited by twistedsymphony: Sep 26 2006, 10:24 AM