Alright, I've got a Gallien-Krueger 700RB head that puts out 320 watts at 8 ohms. If I bought a Gallien-Krueger 410BLX cabinet thats impedance is 8 ohms, and handles 400 watts, would they work together, with no damage to either, or any problems?
Yes, you're putting 320w into a cab that can handle 400w, you should be fine.

Turn it up in the store though, and test the rig before you buy it.
"You can practice to attain knowledge, but you can't practice to attain wisdom." - Herbie Hancock
Got another question: The head will operate in 4 Ohms, 8 Ohms, etc. The cab handles the wattage at 8 Ohms. What do I need to do to get the head to go at 8 Ohms? Is there a switch I need to hit or something, or will just plugging them in together make the amp go in 8 Ohms?
ACtually there is a school of thought that reccomends OVERPOWERING your speaker.

The worst thing you can do for a speaker is run a clipping power amp into it. if your amp has too little power there is a good chance you will overdrive the power section & possibly wreck your speaker. if your amp can put out more power than your speaker, this will never happen.

If you get another 8 ohm cab, you can run them both off that head & it will effectively be runnig at 4 ohms