This is just my own research into when upgrading from the 300/350/400D whether to go for the 20D or the 30D. My own conclusion is save money and go for the 20D or go the whole hog and go for the 40D. These are my thoughts only and the articles are freely available on numerous web sites
Upgrading from (or to) and EOS 20D or 30D?
For those upgrading from a 6MP DSLR like the EOS 10D or original Digital Rebel both the EOS 20D and EOS 30D represent a significant improvement. Both the 20D and 30D offer a higher pixel count, faster operation (including very fast "wake up"), better AF and overall better usability. While the EOS 30D is a better camera in terms of features, with the EOS 20D selling for around £150 less it's still quite a viable alternative. If money is tight and image quality is your major reason for upgrading, the 20D might just be an excellent camera for you while it's still available new. Both the 20D and 30D are worthwhile upgrades from the original Digital Rebel, EOS 10D, D60 and D30. Not only is the pixel count higher but camera operation is faster, AF is better, the shutters are more durable, the sync voltage is higher and they are generally nicer and easier cameras to use. Upgrading from a 20D to a 30D is principally a features upgrade. Both cameras are capable of yielding the same image quality as outlined below.
Canon EOS 30D or EOS 20D?
The EOS 20D is a very good camera indeed. I've been using one for over a year and I've been very happy with it. Good resolution, low noise, very good quality. Even though the 20D and 30D share the same digital sensor, the 30D does have some nice additional features. A larger LCD screen which, from my experience with the EOS 5D, is significantly easier to read (especially if your close vision isn't all that it once was!). The firmware improvement such as Picture Styles, and RGB histogram, tweaked AF etc. are also a definite plus, as is the addition of a spotmeter. Though these extra feature don't change the intrinsic image quality, they do make the camera easier to use and features like the spot meter and RGB histogram allow the advanced user to more tightly control and analyze exposure.