The difference between the Nikon, Canon and Pentax isn't all that great, they will all give very good results. Canon DSLR's have the best noise performance and that is a plus for Astrophotography. The 350D is also 8Mp (as against 6 Mp of the Pentax and Nikon) and the extra resolution is useful, especially when cropping images. The 8Mp E300 Olympus reportedly doesn't perform as well at high ISO's as the other DSLR's.
