Well, I know how you feel Fabsroman, it is a little confusing, and a person hates to spend a lot of money you don't have to.
Ingerson Rand is deffinately the Cadillac of the compressors, and I wouldn't own anything but I-R air tools. But for the budget minded individual, I would recommend Campbell-Hausfeld. Thats what I own. Check out this website.
WWW.NorthernTool.com
Or like I told you about the Alemite products, about any good auto supply store should be able to get Campbell-Hausfeld. I bought my compressor from the local Napa store. It is a 6 1/2 hp motor, 60 gal tank and paid about $550 or so back about 8 years ago. My compressor is a single stage. It has done me well and I have run an orbital sander with no problems and impact wrenches are deffinately no problem.
I called a friend out at the local auto parts store for some help with this. He explained to me that a two stage compressor has one piston bigger than the other. The first piston compresses the air some, the second piston compresses it more. Instead of a 60 gal tank having 60 gal of air, it is like haveing 120 gal, if this makes sense. He said that the average homeowner, or even repair shop doesn't need this compressor. But, if you were going to run it all day long, 5 or 6 days a week, in a body shop running sanders or sandblasters, you might need it though. He said they had professional garages using the same compressor I am using. The price is now $575.00.
Hope this all helps you. If you need anything else, give me a shout.