HD 4870 x2

No it won't work..

because the 2009 mac pro and mac pros in general lack another set of pcie connectors to easily install a 2nd card, unless you get some kind of booster with its own power supply.

same story, sorry. I have the 2008 model. Now, you could use two GT120's, but you know this card stinks when it comes to performance.

Can I use any 4870 or is there a Mac specific SKU I need to purchase?

You need a Mac version
<http://store.apple.com/us/product/MB999ZM/A?fnode=MTY1NDA5OQ&mco=MTA4MzU2NTc>

And more importantly : Will I see performance boost on the 2008 ?

Yes.

I think he's referring to using a Radeon 4870X2 Dual Core GPU ( http://www.pugetsystems.com/blog/wp-content/uploads/2009/04/4870x2_internal.jpg), not running two normal 4870s.

If that's correct, there's a chance it would work in any Mac Pro, it just needs to have both the 32-bit and 64-bit roms like like a normal 4870. You could very likely get someone to flash it for you, the only thing is, as with CPU cores, it's up the OS to support them, which I don't think OS X does right now. I haven't done a lot of research on this, as it's pretty new, but in the end even if does work, I don't think you would see any benefit on a Mac.

JM

I think you got the run-around.

It works, but no video out in Mac OS, works in Windows.

If you plan to use Mac you'd have to cripple it by flashing, unless there is a non-flash injector method (and have another card provide early boot video).

As long as it works with the two 6-pin PCIe aux power connectors you are fine.

There were some threads about the x2 on MacRumors:
http://forums.macrumors.com/forumdisplay.php?f=1