To answer this question you need to give out more info. I just recently started an A+ class, it is all about hard ware and how to install and configure operating systems, and one of our first classes we built up and tore down a 200Mhz computer. I finished it under 30 minutes. Of the suggestions I can give you is to become familiar with you motherboard manual. If you don't have one search on line for manufacture for support. One of your potential problems is with the size of your board. Boards most often are AT, baby AT, or ATX form factor but some manufatures make what is called proprietary boards. These boards have risers boards comming off the main board. These boards are made for that particular case. So you can see the potential trouble. These will often have onboard video card which might be inactivated by jumper settings. I cann't make suggestion on these boards because I have limited experience in computers i.e. 2 months. But if you have a board that is form factor AT, baby-AT, or ATX you should have limited trouble with a new case. Reference that I have used have been www.mcp.com, which has many books available free to read on line. A word of caution is that static eletricity can easily fry your hardware so use a five dollar wrist strap avaliable through compUSA that give you some protection from frying your hardware. Another suggestion is to document what you are doing when you disassemble the computer. You don't want pieces left over. Remember to look at the ribbions that connect your hardware to your motherboard. You will notice a red stripe on one of the wires, this is the #1 wire and it needs to be connected to the #1 connector space on both motherboard and hard drive or floppy. These #'s are usually on the board or device. albeit small. Also notice on the ribbion that connect your HD? may be floppy that there is a twist closest to the device this is important and should be duplicated.
Good luck! Trying to explain to others helps better understand it I hope I didn't give you any incorrect info.