Howdy
If u can, the next higher Canon model the 20D may be worth considering, as it turns off its sensor amplifiers during exposures, eliminating "Amp Glow" on edges of images.
This website is excellent for showign what amp glow is
http://www.tcann.com/
See the 20D dark frame, totally even across it. Mind you, using Gradient Xterminator photoshop plugin is good for removing amp glow, of if the object being imaged is smaller, just crop it out.
I wish I could own a 20Da, its a new model with a different IR cut filter that allows mugh more H Alpha red night from nebulae, its actually the only DSLR marketed as an astro camera, not yet available in Australia.
Scott