I have 2 5870's in my Windows machine. A few things you might want to know (I didn't).
1 - The 5870's usually have 2xDVI, 1xHDMI, 1xDisplayport. However the card only has 2 timing chips (I think that's what they are called, let's pretend they are). The only output that does not use a timing chip is the Displayport. This means you can only use 2 of the 2xDVI, 1xHDMI. So either 2xDVI or 1xDVI and 1xHDMI.
2 - The displayport does not need a timing chip if used natively. So if you're plugging this into a displayport monitor you're fine. However if you're plugging this into a DVI monitor using an adapter (Displayport to DVI) you will need what is called an Active Displayport adapter. This provides a timing chip for this "new" DVI port. These run about $100 (they have a USB dongle that plugs into your PC). The most common one is sold from Dell. That being said they are known to be hit or miss for working. You may end up returning them for a new one until you get one that works. You'll know it doesn't work when your displayport -> DVI monitor just starts flickering off at random.
3 - Eyefinity is not supported across 2 cards. So you can't run 2xDVI off one card and 1xDVI off another card. Yet. There are rumblings that they might support it in the future.
As for it working under OS X, it currently does not. It should work under Windows but my cards are in an actual Windows box. Some games are not that great under Eyefinity to be honest. For instance Dragon Ago looks cool but you can't move the UI so all your casting bars and character portraits are on the far left monitor and the tooltips for the spells are on the far right monitor. I'd highly recommend you look into what games you want to play before you go this route. I've almost given up and play most of my games on one monitor these days. If you have a question about a specific game I might be able to answer... I'm a Steam addict...