Basically, I had a terrible experience with SanDisk Extreme III cards. If you missed the thread you can read it
here.. Anyways, the problem is the Extreme IIIs require a minimum write speed from the buffer to the card that the 20D doesn't quite have. (although SanDisk is the only people who know this and they don't really tell you. It took me hours on the phone with them for to tell me what the real problem was. Canon actually doesn't tell what the read speed is from the buffer to the card. I'm not sure why....)
ANYWAYS....the moral of the story is, if you stick with anything other than the Extreme IIIs you should be fine (for the 20D, the Mrk II should handle the Extreme IIIs just fine.)
SanDisk sent me a bunch of replacement regular extreme cards. These are a little better than the Ultras. I would stick with these if you want to get some. They are made to last a little better than the Ultra IIs. Either really should be fine though.
I actually have 3 Lexar 2 gig cards as well. I may try to sell them, cause I really kinda want to stick with one company rather than both. No real reason honestly....I just like the idea of sticking with one company! I'll probably throw up some of these Lexar cards for sale at some point on the forum if anyone is interested. If anyone wants them right away just PM me and we'll see if we can work something out.