Steve Dunn , Jan 03, 2006; 04:40 p.m.
It seems Canon isn't fond of stating the maximum card size their cameras can take; they are fond of saying they can use cards 2 GB and larger (the manual says so, as does the Technical Report in the Canon Camera Museum), but I don't believe I've seen an official statement of what the upper limit is.
It can definitely use 4 GB cards; Rob Galbraith's database shows the performance results of several 4 GB cards in the 20D. I would be surprised if it couldn't use 8 GB cards as well.
That said, there are arguments for and against using one large card as opposed to a number of smaller cards. If you have multiple cards and one of them goes bad (or you realize you just deleted an image that you'd like to undelete), you put that card aside until you can attempt recovery, and put another card in so you can keep shooting. You can't do that if you have only one card. If you drop off your card at the local photo lab to have them make prints, you don't have to wait for that card to come back if you have multiple cards. Of course, in favour of one large card is convenience; you can take a heck of a lot of pictures on an 8 GB card before you have to pause and swap cards.