You want to "just insert a new MB and CPU" into the case?
Maybe I don't understand correctly. The processor is *in* the motherboard. As are the sound card, video card, the RAM, the modem, the network card, extra USB ports, and any other PCI cards you have in there. So you can't simply insert a motherboard into the case. You'd need to take out all the IDE cables, all the wires going to the power supply, all of your PCI cards, your AGP card (which you may or may not have), your fan/heatsink (which, mark my words, will be a ***** to get out), and then your processor.
Then you will take your new motherboard (Asus, Abit, AOpen are the best), and insert the new processor into it, then put the fan/heatsink in (which will, once again, be a ***** to put back on). Then screw in your motherboard to the case. Reinsert all PCI and AGP cards. Reinsert RAM (which you may need to get a new stick or sticks that fit your mobo). Reconnect IDE cables and power supply cables.
Also another thing to worry about is the little cable/wires (I forget the name right now)... they are attached to your case, and you need to put them into the motherboard in their respective places. These turn on the case light, turn the computer on, and make the mobo make sounds when/if overheating.
And if you are getting an Athlon, you must connect the fan/heatsink before you boot it up. These things release so much heat that you can fry your processor in only seconds. Be careful and make sure you know what you're doing.
Another thing... I'm not sure if changing motherboards will basically act as formatting your hard drive. I believe everything will go bye-bye, but I'm not totally sure on that. Ask somebody who knows, or back everything up.