HA ~ actually, the camera is a start.
Choosing a len within budget is more difficult
I have 450D and 7D.
I don't think 450D is a good choice in bird photography unless you have a good len, such as 428.
As the above Ching said, 450d has only 3.5fps. One more thing, the shutter reaction is very slow (forget~ 450D: around 100ms , 50D: around 55ms).
If choosing between 450D and 50D, 50D is much much better especially in motive photography.