You might have a hard time making that combination work at all and/or work reliably being as Eye-Fi cards are SD cards, and the Canon 20D takes CF cards. A few people I know have tried to use SD to CF adapters but their results have been mixed, at best. On the Eye-Fi site they say the following about using SD->CF adapters:
"Eye-Fi does not support the use of SD to CF card adapters with the Eye-Fi Card. Eye-Fi has not tested the Eye-Fi Card in cameras designed to use CF cards and has no explicit knowledge to share about the success of these adapters when used with an Eye-Fi Card. We only support the Eye-Fi Card in cameras designed to use SD or SDHC cards.
We are aware that many users want the Eye-Fi Card functionality in their CF-based cameras and have opted to use a CF card adapter to get the functionality offered by an Eye-Fi Card. The following list of known issues with CF card adapters is a collection of information gathered directly from customer and blogger descriptions of issues they have experienced. By sharing this information Eye-Fi accepts no responsibility for problems encountered when using the Eye-Fi Card and a CF Card adapter.
- Wireless range of the Eye-Fi Card is noticeably reduced.
- Formatting the Eye-Fi Card in a CF adapter has caused the Eye-Fi Card to fail.
- File corruption of photos.
- The Eye-Fi Card will not work in Canon 20D, 30D and 40D cameras when used with an “SD/MMC” CF adapter. Use of the newer “SD/SDHC” CF adapters appear to work with the Eye-Fi Card in these cameras."
If you need something more reliable than the 'it might work, but if it doesn't you're on your own' statements from Eye-Fi, and you have a stack of money to burn you can always go the route of buying the Canon WFT-E1 wireless transmitter... It's about as robust as a wireless solution can be, it's an official Canon product, so its supported by Canon, and it's about $900. Ouch.
Now if you're willing to set aside the requirement of 'wireless,' then it's actually relatively easy to get the functionality that you're after, and for a lot less than $900. A simple USB cable to tether your camera to your computer and Adobe Lightroom 3's tethering functionality will give you instant display of any image you take. You can get ~16 feet of play on that USB cable and the setup (assuming you're not already a Lightroom user) will run you about $10 for the cable, and $250 for Lightroom. If you need more than 16 feet you can buy cables with 'signal boosters' built in (such as this cable) which can get you even more distance.
I've used the Canon wireless transmitter setup in a friends studio and his feedback was that the system was great, but expensive and more 'fiddly' than he would have liked. I use the tethered setup I describe on a daily basis in my own studio and it is rock solid, easy to setup and maintain and super convenient. Soooo...
Unless you absolutely need to be able to shoot 30+ feet away from your workstation you'll probably find the tethered solution to be more cost effective and reliable. On the other hand, if you've got a studio space that is large enough that you need to be 30+ feet away from your workstation than probably the $900 for the wireless option isn't that much of a stretch for you anyway... That and the assistant you'll want to hire to keep everything running smoothly. ;-)