I've just assembled a 5 gpu rig. Granted, 5 x 280x but the only driver I could use to make it work was this: https://litecointalk.org/index.php?topic=11761.0

Followed the instructions but I had to update the drivers twice, dunno why. Be prepaired for a lot of screen flickering/shutting off for a good while when updating the drivers at itll go through all of the cards.

Needs to be noted though, no idea if this'll work for you with 270's and 290. But it looks like it could.

Edit: oh yeah, im on windows 7 x64