Here’s a quick video that explains some of those connections:

www.youtube.com/watch?v=pevq0um0Bn8

There are only 2 exceptions that I can think of where you can get things turned around with negative (but not drastic) results, and that’s the sound card connections and the PS/2 ports.

The PS/2 ports are the 2 small round ones with several pins in them.  Your mouse and keyboard plug into these.  Typically they are color coded, but not always.  There is usually a symbol next to them on the back of the case indicating which one goes where.  If you get them switched, no harm done, they just won’t work right.  Just turn the computer off, switch them, turn the computer back on, and they should work fine.

I might add that the connectors need to be turned the right way before pushing them in.  Never force them in.  If they don’t seem to go in relatively easily, inspect the pins to see if any are bent.

Here’s a diagram of the connections on the back of a Dell Dimension: