Crossfire is for AMD cards, SLI is equivalent to Crossfire but for Nvidia cards.
Cant really answer that question, as it is very subjective to what games you play. Some games are very well set for multiple GPU's while others aren't so well optimized and suffer for it.
Also theres no point Crossfire/SLI'ing lower end cards when you can just buy a single card of equivalent strength and price. Only worth doing Crossfire/SLI when your talking higher end cards or if you already own a card.
Also we need more info. If your after advice on which card to get, we need to know your budget and usage.