.... hmmm not so fast....
The 20D is a great camera for sure. I should know I've still got one, but it is now 3rd generation. Superceded by the 30D and now 40D. The difference between the 20 and 30D is not that great, but the 40D... wow!
The 400D has some of the newer features and capabilities that make it very interesting to someone starting out with DSLR cams. Including a 10Mpix sensor, and cleaning.
Plus its smaller and lighter and you can buy them new. That means a years guarantee (or more).
20D's are all going to be 2nd hand now. A good one should serve you well, but one that's been hammered .... could cost you more getting it fixed than its worth.
Still you can get one a LOT cheaper than a 400D new.
Which would I go for. Well the 20D, since I know it well and it suits my big hands, but if I bought one as my primary camera I'd be saving like billyo to get a 40D.