Mav,
Without looking at your system, you have a P3, you most likely have have a ATX case. I think you would be able to just buy a new motherboard and use all the components out of your existing computer. I have used many motherboard brands, I prefer Asus but whatever you choose go to their website and make sure it takes your chip, proper slot type, ram type and is usb. If your current case is not an ATX, cases are cheap, just get one with a 300 watt power supply.
Big Mac