1. Not yet. SATA is just another way of connecting a HDD to the system. It is still somewhat more expensive than ATA. It is faster, but all HDDs are much slower. The advantage is that the cable is better (less airflow obstruction, easier to handle and max. cable length is longer). It is probably going to replace ATA in the future. Most mobos with SATA support only 2 devices. I haven't seen any ATAPI devices yet.
2. Only one. You might want to attach multiple drives in a RAID 0 configuration for very fast drive access. You might want drives in a RAID 1 config for increased security against drive failure. You might want a very fast (and expensive) system drive and a very big (but slower) drive for video purposes.
3. RAID = redundant array of inexpensive disks. It basically bundles your drives into one logical drive. RAID 0 (aka striping) spreads the data over multiple drives, therewith increasing the read/write speed, but at the cost of security (if one drive fails the whole array fails and all data is lost). Raid 1 (mirroring) basically keeps an identical copy of a drive on another drive, so if a drive fails the system keeps running from the other drive. This costs you half the drive space (2x80GB drives = 80GB space). RAID 5 is an improved version of raid 1; it will cost you only 1 drive per array (it will cost you 1 drive on a 2 drive array, which is not much of an improvement to RAID 1, but 1 drive out of 4 is). All RAID modes require you to utilize drives which are of about equal space (or better said: the smallest drive will determine the available space). Last, although technically not RAID, JBOD (Just A Bunch Of Drives) which is basically one logical drive spanned over multiple physical drives. An alternative to RAID 0 if your drives differ in capacity, but without the performance gains of it. You need a RAID controller to implement RAID.
4. Athlon systems have a better price/performance than Intel. Very popular are mobos based on the NForce2 chipset. The Athlon XP 2500+ or 2700+ have an excellent price/performance ratio. You need 2 DIMMs of PC2700 DDR RAM, PC3200 gives you better performance. Maxtor or Western Digital 80 or 120 GB drives are nice drives.