Recently, ATi has started releasing drivers for CrossfireX, a technology which allows very convenient and casual mix-matching of cards for use in a multi-card configuration. A 3870 can roll along with a 3850, or even two 3850s, for example. Indeed, CrossfireX allows for up to four cards. When people speak of quad SLI, they mean two dual-chip cards. When you have quad CrossfireX, you have four full boards.
A nice chart of Crossfire compatibility can be found here, but ATi has yet to include the 4 series on that page. If they follow through with their preliminary promises as reported by numerous news websites, we should also be seeing “inter-generational compatibility” between ATi cards. You got it, that means you can have a 3850 and a 3870 right now, and go out and toss a 4850 into the mix if it tickles your fancy. I'd like to see the brave site that tries to document scaling between all of those possible combinations!
Setting up Crossfire is really easy, but there's not really any documentation on the hardware part anywhere. First, you need two cards that are compatible, which means any two of the cards from the new naming sequence (HD 3850, HD 4850, HD 3870, etc…). Two cards in Crossfire require two bridges. Unlike nVidia, with their single bridge design, ATi uses two, which makes it much easier to expand to three cards, and eventually four if that's your thing. Because every card comes with one bridge, you will always have enough. Just in case, though, most Crossfire compatible motherboards come with a couple extras. Installing the bridges is as easy as slipping them onto the card's tooth (while the computer is off!) and it doesn't even matter if it's upside down or not (the J1 side can be on the top or the bottom card without changing anything).
After you've installed your two cards and two bridges and made sure that both cards have the proper power connections, the next step is to boot up the computer. If you've had a single card and you're upgrading to Crossfire, then your drivers are already in. If this is your first build, and you're going straight to double cards, then you have to install drivers just like this was a single card installation. Those of you already familiar with the Catalyst Control Center (CCC) will notice that a new tab has mysteriously appeared in your CCC sidebar: Crossfire. In this tab is a check box to enable Crossfire Technology. Once you do that, you're done, and you can go on gaming with the benefit of two cards working together instead of just one lonely red PCB (or none). ATi explains in detail the software part of setting up Crossfire here.
With three cards, no new special bridges are required (like they are with SLi). Instead of having some sort of bridge that has three connectors instead of just two, you place one bridge connecting the first card to the second along one tooth, and then another bridge connecting the second to the third on the other tooth. Adding a fourth just means adding another bridge, continuing the zigzag pattern.
Now that we've all got a nice background in Crossfire, let's get on with the testing, shall we?