You will certianly be able to get another 590 with your MB as you have two pci-e slots running at x8 to accomodate the cards and your board also supports quad sli. I don't see where your cpu would be a bottleneck.
I figure by the time youll feel the need that its time to add a second 590, there will already be much more efficent, powerful single cards out that youll want to get instead. I mean, there came a time I had a 9800GX2. By the time i felt i needed another 9800gx2 we were up to the GTX400 line.. My personal opinion of course and im a single monitor gamer in the 1900x1200 range, your needs may be very different than mine..... And yes, your mobo is listed as having quad sli-abilities.