There is not that much choice...
Best candidates on a low budget are the Nikon 70-300 VR ED ($700?) and the Sigma 135-400. The Sigma is surprisingly good at 400mm for a lens below $600.
If you have more budget, the Sigma 50-500 and the Tamron 200-500 (both below $1000) are worth considering. Also the Sigma 80-400 OS (image stabilized) is nice option which still is below the $1000 mark.
Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30