I’m an amateur astronomist, or even less…
I’ve been using Stellarium on my 15″ Powerbook (G4 1,67 GHz with Radeon 9700 – 128 MB VRAM) quite thouroughly, and trying it on my Windows box as well.
It may well be one of the best sky software ever: for sure it’s one of the best way to look at the sky – apart from staring under the night sky trying to spot each and every star, planet, constellation and so on.
Stellarium is an open source desktop planetarium for Linux/Unix, Windows and MacOSX. It renders the skies in realtime using OpenGL, which means the skies will look exactly like what you see with your eyes, binoculars, or a small telescope.
The Stellarium project was started by Fabien Chéreau during the summer of 2001 and uses Sourceforge intensively.
Stellarium is very simple to use, which is one of its biggest advantages: it can easily be used by beginners.
It has an extensive features list, a huge objects (Deep Sky included) database, lots of detailed close-up images of stars provided by users/astronomers…
Just go & try it, not because it’s free: because it’s great!