AHHHH so many questions! Heres my 2 cents:
1. bus speed - how fast info travels from chip to chip etc on the motherboard.
2. bus - a path the data is transmitted across
3. IDE is here to stay! Integrated Drive Electronics aka Advanced Technology Attachment (ATA) or Enhanced Integrated Drive Electronics Started as a 16bit bus standard made by IBM i believe.
4. There are RAM limitations on Mobos.
5. Yes you can have more than 3 hard drives just not IDE, RAID is a good way to utilize alot of harddrives. 3 harddrives and a cd writer is all 4 of your IDE slots and i think you can only have one master harddrive and the other 2 would be slaves, but each chain has to have a master. Someone will correct me on that im sure, sorry
6. I got a really nice Barebones system that runs AutoCad flawlessly from www.econopc.com
for a really low price and i havent had a single problem. www.tigerdirect.com
is good too. You'll need a good video card and alot of Ram with a decent system backing it should be good enough for most programs.
7. Cache is a "little library" that holds information you access frequently so it takes less time to load. Cache comes in many levels and variations.
Hope this helped!