I would personally get the Sandisk Ultra II cards. I have three of these and one Sandisk Extreme III. I have had such good luck with them that I will probably buy another before I go to Colorado in September. As I posted in your other thread Canon does not make their own memory cards. That's not to say that they are not any good, I have not used them but most pro-photographers use either the uper end Sandisk or Lexar cards.
OK, I just checked the B&H website and you can get a 2gb Sandisk Ultra II for $5 more than you can get the Canon 512mb card, or get a 1gb Sandisk cheaper than the Canon 512mb and the Sandisk is a better card. The B&H site under "Features" for the Canon it discribes it as a "standard" speed card. THe Ultra II is faster and the Extreme is faster still.
For more info on CF cards check out this site read the page thaI linked to, then be sure to check the drop down menu for the 20D.
He does not test the Canon but he does test the actual makers of most cards.
http://www.robgalbraith.com/bins/multi_page.asp?cid=6007BruceP
