Check out this discussion thread on dpreview:
You'll find other discussions on this topic on dpreview as well. Basically, the opinions are split 50-50. It boils down to a personal decision.
I mostly sell on eBay and have been a member for 9 yrs. Before buying from anyone, check their feedback score. If it's less than 100%, read some of the feedback to see who your buying from. Most important. Do they have a return policy? You can get good deals on eBay, but just be consumer smart. If someone is offering a kit, pick it apart and look at the individual prices of items in the kit to see if your really getting a good deal. The main difference between eBay and Amazon is that Amazon has a very good customer service and stand behind what they sell with a good return policy on many items. Ebay, well a lot of sellers on eBay do not have a return policy, and should you buy something that is not as described, you might have a hassle working it out with them, and have to get Ebay customer service involved. I say this from experience in buying from both. Again, just be consumer smart. Good luck!
There is no "correct" choice, it is all up to you. The 30D is a good camera at a lower price, but uses previous generation technology. The 40D is the 30D's replacement and has the latest technology and higher cost. If your more concerned with money, go with the 30D. If you are more concerned with having the latest technology, get the 40D. Beyond that, look at each model and choose based on the features you want or need most.
In general, the lens will make a much bigger difference in the quality of your photos than the camera body. So, if money is an issue, its always better to go for the cheaper body and spend the money you saved on better lenses.
I got cheep aftermarket batteries because I got mine used and the Cannon batteries wouldn't last that long. The cheep batteries work great, and I can shoot for hours without a recharge. I got a bigger memory card, and that was nice. The flash on the 40D isn't that great, so consider getting something that will let you use the bounce technique. Lenses are all about what you like to shoot, so it'd be tough to advise, but I like the prime 50mm f1.4 Cannon lens for photographs of people.
I had the 20d I upgraded to the 60d. I really like the quality of the pictures. I'm a wedding photographer and it is so much easier to use, it has the Q button on the back for quick access to all the setting options you can change.
How much are you willing to spend? I'd suggest the $1300 Canon EF 100-400mm f4.5-5.6L IS USM as a very good general purpose wildlife lens. For birds in flight or smaller birds, this might not be quite enough. Otherwise, depending on the image quality you want (check FM Reviews at http://www.fredmiranda.com/reviews/ ), how much you're willing to spend and how much you're willing to crop, you might also look into the following lenses:
Sigma 70-300mm APO $200
Canon EF-S 55-250mm f/4.0-5.6 IS $240
Canon 70-300mm IS for about $500
Canon 70-200 for about $500 or $1000 with IS
Sigma 120-400mm f/4.5-5.6 AF APO DG OS HSM $750
Tamron AF 200-500mm f/5.0-6.3 Di LD $800
Sigma 150-500mm f/5-6.3 AF APO DG OS HSM $900
Sigma 80-400mm f/4.5-5.6 EX OS APO RF $900
Canon EF 300mm f/4L IS USM $1000
Canon EF 100-400mm f4.5-5.6L IS USM $1300
I'd also suggest reading some of the birding forums before making your decision.