PCI slots are for things like soundcards, network interface cards, etc.
SLI is Nvidia's dual card technology and Crossfire is ATI's. You may use a single card of either brand on both types of motherboard.
Yes, SLI and Crossfire means that you can run two cards as one, increasing your video gaming performance.
Your optical drive can be either IDE or SATA. They sell them in both flavors.