I was interested in your question and in the replies, since I just returned from that region a couple of weeks ago.
Based on my recent experience, I think you might consider what you would plan to do with any photos that you take. If you expect to make gallery-sized prints of images of distant wildlife, you’ll need an appropriate, heavy, and costly kit (including, likely, a very sturdy tripod); if you want to view images on your monitor to help you to remember the experience and to make some modest prints, you can get away with carrying much less.
Let me give you an example. I have an extremely sharp, somewhat interesting, head-and-shoulders image of a moose that decided that he, rather than we, had the rights to a trail in Grand Tetons National Park, just south of Yellowstone. Since I’m not really interested in hanging a 16 x 24 inch photo of a grinning moose’s face my wall, the images that my daughter captured with her little Olympus XZ-1 are every bit as good for our purposes. OTOH, I have some images in which it’s just possible to tell that the object near the center of the frame is a white wolf; I suppose if I’d had a proper telephoto I might have a better photo of a distant wolf, but I still wouldn’t do much with it other than zoom in a bit to look at it on my monitor, since the wolf wasn’t doing anything particularly interesting and it wasn’t far from midday when I happened to see it. Nonetheless, since (unlike 97% of the people who visit Yellowstone) you’re willing to venture more than a few hundred yards from your car, you might see something interesting.
I happened to see something interesting, and I didn’t need exotic gear to capture an image. Since I’ve been freaked by grizzlies ever since one tried to take my food while I was backpacking in the park, I took a photo of a bear paw print that I happened upon one morning. Since it had rained the day before and then snowed that night, it was clear that a hiker had passed that way an hour or two earlier, a mother grizzly had made much fresher prints that overlay the hiker’s, and her cub had added its marks just behind mama’s. (That was about 1/3 mile from the Old Faithful Inn.) I’m not going to hang it on my wall, but my kid put a 4x6 inch image in her album, and the photo will help us to remember the experience. (BTW, a grizzly killed a hiker in Yellowstone today; he’d unexpectedly come too close to her cub.)
FWIW, I used a D300 and a 24-70/2.8, along with a 105 macro lens, and a 1.4x teleconverter I brought along to allow my child to enjoy taking photos of critters. I hope that the next time I go I will have a Nikon 70-200/4 (!) instead of the serviceable but limited short tele set-up. I also carried a tripod, which was useful for a variety of reasons, not least because taking multiple exposures allowed easy erasure of some of those 950,000 or so people who crowd the usual destinations in the park during each summer month.
