You're thinking of the different rendering modes. There's AFR, weighted sharing (or something like that), SSR... I can't recall them all or what they're referred to as. End result is, you share the rendering load between two cards and get a performance boost.
How great a boost varies widely -- anywhere from around 70% to 30%. Very rarely is it actually "twice as fast" as a single card, but that's not the point. SLI and CrossfireX are only really worthwhile on extremely high resolution (1920x1200+) screens where a single card just isn't enough to maintain high settings and reasonable framerates.
If you're only looking at 20" and 22" displays, which tend to be 1600x1200 for standard aspect or 1650x1080 for widescreen, you really don't need to worry about SLI or CrossfireX, as they probably aren't worthwhile for you. A single card should do just fine at that resolution.
It's worth noting that SLI and CrossfireX require compatible motherboards with a couple of exceptions. There's the Geforce 7950GX2 from Nvidia for a past example. Upcoming cards like the Radeon HD 3870X2 and Nvidia's 9800GX2 won't require SLI or Crossfire capable boards because they are CrossfireX and SLI "on a card", respectively.
I have read somewhere that the 3870X2 will handle the crossfire scaling within the card. I think this will work on any x16 PCI express slot, whether you have crossfire or not. Correct me if I'm wrong though.