Re: hmm some important questions
Your upgrading basically everything inside that computer? I'd reccoend getting a new case. The old one would probably work, but i don't know about that power supply. They've gotten more powerful and processors require more juice than before.
When you say slot...do you mean PCI slots? That all depends on what you want to upgrade. Most mobos have decen soundcards, NIC's, etc. about all you need to worry about is what kind of graphics you want. As long as the board has an AGP slot, you pretty open to whatever you want. PCI-Express is starting to pop up, but I don't think the cards are much better than the AGP 8x cards ATM...