Hi Pete,
Well, it depends on which DSLR you choose and the lenses. If you go for the new Canon 10D, then the body alone goes for $1,500 (U.S.). You are right, for good quality A3 prints, you do want a DSLR. The 10D is very highly rated. The Canon D60 and Nikon equivalent (can't remember the number), each body is between $1,770-$2,000 (U.S.). Figure another $150 for batteries and car charger (assuming you want more than one battery, this will give you three, at least). Then of course the media cards, depending on what you already have. You will want a minimum of 512 mb of storage, 1 gig is preferable (not necessarily all just in one card). Then comes the most expensive part, the lenses. If you can get close enough for a good shot of bird with a 100-300, or 400mm lens, then you can spend anywhere from $500 to $2,500. I highly recommend you get IS (image stabilization), where available. You WILL notice a difference. However, to do it right (my definition) you really need the big 600mm IS lens, and that goes for $7,500. All prices are new equipment. So, in the end, figure a ballpark figure (adding the 600IS lens) at about $10,000. Without the lens, figure on a ballpark of $5,000 (this is with the 400mm IS lens).
If you are talking about selling, then I recommend only print them professionally. However, I would recommend a good printer to print up test shots first, remembering they will only be magnified when done professionally. I have the HP 1100 (only about $150 U.S. now), it prints very well. When I print it up and notice noise, speckles, blurriness, CA, etc., I know I better take care of it because it will stand out like a sore thumb when printed up professionally. Mind you, my camera is only 2.6 mp, but I have found it very useful to do this. You can see why I have not upgraded by the $10,000 price to "do it right." But, I have definitely looked into it more than once.
Oops, almost forgot the tripod. Plan on spending anywhere between $200-$600 for a worthy tripod and head. I have a Manfrotto, wouldn't be without it.