Phil's Rambling Rants
My longest lens is a 200 f/4, and I generally wish it was faster---longer would often be good, too. Thing is, long fast lenses are heavy and expensive. A 600 f/4, as Philip Greenspun always says, costs and weighs as much as a good used car. You ought to look at Phil's photo.net, if you don't already know about it. There is some long lens info in there. My SLRs are manual-focus Nikon and I'm not too familiar with the modern gear.
If you don't have a good tripod and head (maybe a monopod, too), that's something to look at seriously, too. Long lenses and tripods go really well together. I have a decent Bogen leg set (3221?) and a (model 410? something like that) geared head. I do admit to feeling slightly silly putting a tiny $350 digicam on this big, heavy, $350 tripod (at least the camera is bigger than the quick-release plate), but it is really nice. It doesn't look as silly with the SLRs. The geared head suits my tendency to photograph things that don't move a lot, like rocks, really well, but you might be better off with a ball head for animals. There is a really big difference between a pro-style tripod and head and the cheap junk.