I have no idea what to do, what should i spend on a camera?

So I love photography but I've only ever had a little $90 digital camera. I've used friends/families nice cameras and I don't put them down until they ask for it back. I'm saving for a camera because I've signed up for a photography class and I want a nice camera for that. I've asked a few people for suggestions and I've gotten lot's of different answers
SX 40 Cannon
Cannon EOS Rebel
Nikon D5100 DSLR
Nikon D5000 DSLR
So I've got a few questions on this.
If I've never had a nice camera, how much is ok to spend? I want to spend as little as possible but it's kind of hard.
is it ok to buy used cameras? On Amazon you can buy a Nikon D5000 for around $500 but that makes me nervous although Amazon is pretty reliable. So is it ok to buy a used camera on Amazon? And what's the difference between used and refurbished? Which one's better?
Any suggestions on which one of these cameras is better? Obviously the most expensive one but for in my situation.

Learn to spell *Canon* and then consider buying a camera.

I would look at getting the nikon D3100 kit. Samys Camera is a great place to buy it. They have terrific service and you can trust them completely. For under $600 you are going to get a great camera.

The best type of camera is the dSLR because it has all the manual controls needed to take a picture. All dSLR's are basically the same. They start at $500. Decent secondhand cameras can be bought at around $250 but you have to test it rigorously before handing out your precious cash. Any brand or model will do. Just make sure you study photography to take advantage of the power of the dSLR.