The cable to connect your 20D to a computer came with the camera. It plugs into a USB port. I think you are a bit confused, though, about what people mean when they talk about quick review. Most users download their cards via card readers or, in the case of a laptop, via PCMCIA adapters. Having the camera tethered by cord to a computer is the slowest possible way to download photos, and puts the camera at risk for a fall should you trip on the cable. It also uses up the camera battery power to download this way.
There is software that comes with Canon digital SLR cameras that allows you to take photos with the camera tethered using the computer, but it is cumbersome. About the only people I've ever seen using it are those shopping mall photo kiosks that make instant prints on t-shirts, mugs, key-chains, etc... Using a computer to take and review photos instantly is a good idea on paper, but it is device dependent. That means, the slower your computer, the slower your photo-taking ability. And you would be limited in what you can shoot since the camera will likely be on a tripod no farther away than the length of the cable, and pointed at a single subject. This might work in a studio environment, but I would hardly think it very useful in the field. Also, stray light through the eyepiece will affect metering if it isn't covered up (there's a rubber cover on the factory strap for this).
The 20D is very good at allowing you to magnify a photo on its LCD and scroll around using the joystick control. This is great for checking focus, but depending on how you'll be using the photo it may not be enough for critical shots that will be printed large. Even so, the best thing would be to have a quick way to download the pics to a computer and view them onscreen with the least amount of hassle. That is going to be a card reader or PCMCIA adapter.
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Rick A. Diaz
http://www.mcjournalist.com
The image is everything.
My opinions are my own. I paid good money for them.
See my profile for equipment list.
