The biggest improvements you will get in image quality is by:
a. improving the optics on the camera system
b. by knowing how to use the camera you have to its best advantage and
c. getting to know your subject.
If you can get a bird full frame then the detail will be there even if the camera is a relatively basic one. DSLR cameras are definately the way to go if you are serious about your bird photography as they pretty much eliminate shutter lag which is a killer for inflight photography and the range of lens is only limited by the depth of your pocket
. Personally I wouldn't even consider a cheap tripod, go for the heaviest and most solid one you can afford and that you can carry. My current setup of Canon 1D MkII N, Canon 600mm f4 IS USM lens and Manfrotto tripod with the 393 head weighs in at around 20-25 lbs but it delivers the results so I put up with lugging it around.
Getting the highest resolution sensor possible does allow you to crop if it's really necessary and still have a high quality image suitable for publication or printing. To summarise I would put my priority of must haves in the following order:
1. DSLR camera
2. A fast lens, preferably a prime one but I know that the better quality zoom lens also give good value for money. Aim for at least 300mm
3. A solid tripod.