PATA cards also work. They can be a good value if you can get them from someone that's upgrading, or if you already have PATA drives available.
Also, many of the affordable SATA cards only have two ports for two drives, whereas the PATA cards tend to have 2 ports for FOUR drives.
But at the end of the day, if you're going to buy 4 new big drives and a controller card, go with the SATA. It'll be faster and you can scavenge the drives if the machine dies.
As far as installation goes ... it's VERY easy. Apple's support website has videos and documentation for each specific model to walk you through it.
The "official" number of drives that you can get into the machine is 4. They sit in the bottom of the case (you may need to buy a dual drive bracket for the rearmost bay). BUT with a little ingenuity you can get another drive above the optical bay, one in the zip bay, and another 1-2 on the floor depending on your PCI card setup (plus one in the optical bay if you pitch the DVD). So thats a total of ... *whips out fingers* ... 5-6 additional, so 9 drives total.
I read some mod page post about a guy that did something like that, but google is failing me. I believe that the major issue was powersupply not being able to handle all that draw. That kind of thing also meant custom fabricated mounting brackets.