Rio Rico and some others have already made some excellent points, but I'll throw my 2¢ in anyway.
I think 300mm should do fairly well, I use a 200mm all the time for bird shots, but I try to get as close as I can. If you look through the shots in my gallery all of the bird shots were taken with Vivitar 200mm, Lentar 90-230mm or Sears 80-200mm. Most are also cropped fairly tight too. You can expect a little better reach with a 300mm, but the difference is not great. I've used a couple of 300mm primes and a 75-300 zoom, and was surprised that they didn't show that much difference in apparent reach. I still wanted to get as close as possible.
As already noted, get as close as you can, use a tripod if feasible, and get to know your subjects. I shoot a lot of bird shots in a local park, where they tend to be more accustomed to people being around, so I can get closer. When I'm out in the field I have to sit there and wait a while for things to settle down, then the birds will start to roam around a little, but they will still be skittish and will scatter at any sudden movement. When in the woods you can forget about good lighting, especially in summer, shadows will kill even a bright cloudless day so plan on a good flash.
More often than not a tripod is a hindrance for wildlife shots, they won't be very likely to sit around and wait for you to set it up. If it's not already set up and waiting a bird or rabbit is long gone before you can even open the legs and set it down. 99% of my shots are hand held. Including macros. Also, if you set up a tripod and a bird or squirrel shows up well to the side, it won't wait for you to pick the rig up and move it around...so if you can't swivel it around far enough, you miss the shot. I gave up on tripods pretty quick. I still have a couple and use them now and then when I can, but most days they stay in the Jeep. One exception was the Pileated Woodpecker nest, I could set it up and leave it, since I knew exactly where they would be...at the nest. I still did 95% of those hand held.
Practice with the lens you plan to use on wildflowers. They stay in one place, windy days help you learn to adjust settings to stop motion (a flash helps sometimes too) and you get some great flower shots. At 300mm you'll have to back off a few feet, but I've gotten some pretty nice wildflower shots with a 35mm ME Super and a 70-210 lens. By the time I got into digital not much practice was needed, but I still try to grab a 200mm flower shot now and then just for fun and to see if I can get it to come out decent.
Watch the lighting. Occasionally you will get a good backlit shot, but usually for wildlife you want good front lit subjects. I never shy away from a backlit bird shot, and quite a few have come out good, but I always try to keep the sun behind be or over one shoulder at least.
Take lots of shots. Digital can be deleted whereas film could get expensive. With film I tried to make every shot count. With digital you can delete anything that is not good enough. But NEVER delete them on camera, wait till they are transferred to computer. The LCD screen is good to have, but I find my pictures are often acceptable when they don't look so good on the camera.
At the ranges you'll be dealing with for wildlife, depth of field is not usually a major consideration, but I always try to shoot at least f8 so I get good depth of field, and most lenses will get better pictures at f8 to f11 than wide open. Use your lens a lot, get to know it really well, and you will usually have a good idea what the picture will look like, as long as you get good focus, before you push the shutter. With birds in flight or any animal in motion, forget about auto focus. It will usually keep searching, and you will often end up focused anywhere but where you want to be...the head. This is especially true with birds, auto focus will latch onto the wings more often than not, so try to learn to use manual focus as much as you can. It just works better.
Teleconverters...I have a couple and use them occasionally, but not often. Usually it's when I want to get a shot for ID purposes and I'm not too concerned about image quality. For good quality shots, I use a prime telephoto and get close. Even then I usually don't have time to dig the thing out and mount it so I go ahead and take a few long distance shots and cross my fingers.
Ok maybe that was 10¢ worth...I'll shut up now...
