Hi Phil. Welcome to IIS!
7x50: Magnification of 7. Lens aperture of 50mm. Exit pupil = 7.14mm.
8x50: Mag. 8. Aperture 50. Exit pupil = 6.25mm.
8x56: Mag 8. Aperture 56. Exit pupil = 7mm
9x60: Mag 9. Aperture 60. Exit pupil = 6.66mm
Ah, I'm just showing off my maths.
Really, what you want in a first set of binoculars (well, what I'd want), is the most Field of View and the most magnification and a weight that you can comfortably hold for long periods. I bought 15x70s, and let me tell you, it's difficult to hold these babies still when looking at anything in the sky. If you can, get to a shop so that you can test a few binos out...checking their weight etc...
As for where to buy...I highly recommend
www.aoe.com.au. They have some literature on binos (as well as other stuff) that you'd find interesting to read and learn about. Here is the bino read ->
http://www.aoe.com.au/binocular_guide.html.
Good luck!