The Canon 20D topped out at around 6 MB/sec when Rob Galbraith tested many of the cards currently available back in 2006 or so. Considering that the 30D improved to almost 7 MB/sec using many of the same cards, I would say anything over 6 MB/sec is overkill for the 20D. The slowest cards on the market today are faster than that, unless they are unsold older stock.

As far as reliability goes, I've never had a problem with any card from SanDisK, Transcend, or Lexar. My newer cards are all the Transcend brand. I've used them heavily for years. I follow a couple of simple rules: NEVER insert or remove a CF card in a device that is powered on. SD cards and USB drives are designed to be hot swappable if "ejected" properly before removal. The design standards for CF includes no such provision. Also, rather than "erasing all images" from a card, I format the card in the camera in which it will be used.

The design standard for all CF cards is a lot more tolerant of extreme temperatures than other types of flash memory such as SD cards. SanDisk touts the durability of their cards when used in extreme temperatures and at high altitudes. My own experience shooting in sub-freezing weather is that it is a non-issue with any of the cards I own.

In cold environments battery life will be your primary concern. Take at least two and keep one warm with the body heat inside your clothes while you shoot with the other. You can swap them and warm the cold one back up and it will 'recover' some of the energy it lost as it was cooled.