I think shooting tweety birds is going to be just like it always is no matter what equipment you have, really difficult. From my experience they tend to hang around in thckets and move about a good deal, I don't think it's just to annoy and frustrate us, I think it's because they always are potentially lunch for some raptor. Thus you have to contend with the subject being backlit sporadically and moments later some intervening leaf or twig getting in the way and screwing up the AF system etc. it's just the way it is.
As Javelin said, probably the best bet is a hide like Gustave uses (if you have plenty of time to kill that is), you can try to find a nest and study the behavour pattern, the parents sometimes use a particular approach path, then you could use the pre-focus + hide method, and/or get them at the nest.
Myself, I've sometimes harboured secret thoughts of trapping em and supergluing them onto a branch.
On the odd times I've had time to try this futile pursuit since going digital I really haven't noticed all that much difference between the A100 and the A700, they both can get fooled by a confusing subject movement-exposure-location, although the A700 does have very slightly faster AF I think (in standard, accurate mode, faster in fast AF, but maybe not as accurate).
Although I don't own one I've seen the results and it's on my lust list, the 70-400G I think is a must for that kind of work, the only thing I'm not sure of is how fast it focusses.
The camera is up to you, you can give the A550 a try (I don't own one of them either), but I might be inclined to wait for the A700 replacement myself, that is if you're patient, there is apparently new A700's still available (so they say, I haven't checked).
Greg