Frame filling raptor shots made with a 200mm lens are often of captive birds or involve doing something ethically questionable like baiting them (a recent photo.net "Photo of the Week" is a good example). If you have a feeder or have access to one, then 200mm or 300mm is a possibility, if you have a window or a blind (and a car can serve as a blind if the feeder is accessible to, say, a driveway) that's close enough, or if you set up a remote triggering system.
That said, most bird photography starts at 400mm. I use a 400/4.5 mostly for handheld flight shots and a 600/4.5 on a tripod. If you're comfortable with manual focus, a lens like a 600/4.5 may be less costly than you think.
How close do you have to be? Well, 15 feet is ideal for a bird at rest. This usually involves finding where the birds hang out--favorite perches, foraging spots, watering holes--situating yourself where the light is in your favor, and waiting for a long time and shooting a lot of film. On a full day of bird photography, I typically shoot around 6 rolls of 36 exp., and I'm pretty conservative. I've often been set up next to photographers who shoot at least twice as many frames as I do. I could afford to shoot more, but it's just not my style. I usually toss out about half of the shots I take in the first edit--a fairly normal ratio. I spent a few hours by this stream where I photographed the bluejay, and about half a dozen species came by, but this was the nicest shot of the day. I was using the 600 and a 25mm extension tube at about 15-20 feet.
A blind helps. I haven't really gotten into using a portable blind--eventually I will--but if I'm in an area where blinds are set up, I'll take advantage of it. The goldfinch was at Jamaica Bay Natural Wildlife Reserve, which has a few blinds set up, some with feeders. Really hardcore types will build floating blinds for photographing water birds on lakes and ponds.
Some photographers are great stalkers. Franz Lanting will dance with the albatrosses. Art Morris will slither on his stomach in the sand to get close to shorebirds without frightening them off. There's a guy named Douglas Herr who is active on some of the Leica forums who takes great shots with old Leica R equipment and a shoulder pod, mainly by using careful stalking techniques in the field, looking at the bird obliquely (i.e., not like a predator), prefocusing and raising the camera at the last moment to get the shot like Henri Cartier-Bresson.
Flight shots are tough, but the bird is larger with wings extended. I'd say the ideal distance is around 50 feet. Closer than that, and the bird is usually moving too fast even for autofocus. The shot above is a composite of four sequential slides of the same osprey. You'll notice that most bird photographers don't show too many flight shots. I've been working on that, and I have a bunch that I need to scan and add to the website (go to http://www.echonyc.com/~goldfarb/photo/ and click on the mute swan for the bird gallery).