Brian
Just to give you some idea on a Canon 20D which is the same image resolution as the 350D. Here I have used a 300mm L f4 with a 1.4TC that gives you 420mm. At 800mm double these image sizes.
The first 2 were taken at approx 30 feet
1) full frame
2) a 500 x 500 crop from full frame
The last 2 were taken at approx 60 feet
3) full frame
4) a 500 x 500 crop from full frame
I am just waiting for the day when the kingfisher comes out of hiding and is less than 20 feet away
This hedge is inward facing to a lake so unless I can walk on water there are not many vantage points to get a clear shot.
If you are serious about bird photography price is should be the last of your considerations.
The lens Andy has quoted is over £4K
Choosing a lens is not a simple matter and cutting corners on cost can be a false economy in the end.
Factors to consider.
Image quaility. ( cheap lens tend to colour fringe more than expensive lens. Expensive lens tend to be sharper. These factors are required if your going to crop small sections from the frame or enlarge to A3)
Weight. ( 500 - 600mm f4 lens are not your walk about and snap lens - after a while I've got used to carrying around 2-3 Kilo of camera with a 20D 300mm F4 and the 1.4 TC then another 3 Kilo for the tripod)
Zoom or Prime. ( Prime lens on the whole are sharper than zoom lens and better suited to be used with a TC - Zoom give you more flexiblity but if your cropping in photo editing software the loss of zoom is not a big deal)
Min f stop - compatability with TC ( you may have a requirement for low light photography - You may lose Auto focus when you add a TC )
Auto focus speed ( required to quickly focus on and track the subject)
Build Quality ( more important for the heavier lens - you don't want it falling apart under it's own weight or if you give it a slight knock)
Rob