Let me start out by listing what I use for equipment and why it works for me.
The body I use is a Canon EOS 20D, which gives me what I need to capture what I want to share. It features a rugged magnesium alloy body that has a comfortable feel, 8.2 mega pixel cmos sensor, easy to use layout of controls, 9 auto focus points, 3 auto focus drive modes, and most importantly, to me anyway, 5 frames per second shooting for up to 23 consecutive shots!. This comes in extremely handy not only for flight shots but for getting that perfect pose from a perching bird. Couple that that with AI SERVO Auto focus (Which will continue to auto focus as the subject moves toward and away from you), and you have a deadly flight weapon!

     The lens I shoot with most often is a Tamron 200-500mm F/5-6.3 Di LD (IF). This lens isn't too heavy which is a plus for me because I shoot most of my wildlife pictures hand held (no tri-pod) The length is right for me because most of what I shoot is either real small or a good distance away. The focusing motor is pretty fast and relatively quiet. I can couple this with Tamron pro SP AF Teleconverters (both 1.4x & 2x) With both tele-converters stacked on this lens it gives me a max of 2240mm. The downfall is loosing 3 stops of light and the loss of autofocus as well as the need for a tripod in most situations.

     My "general purpose" lens is a Canon 75-300mm F/4-5.6 IS USM, this lens works great for birds in flight that are fairly close. I also use it for scenics & mammals.