The 590 has 2 580's each with 1.5GB of ram for a total of 3GB. As everything is divided relatively evenly between both 580's the use of the 3GB is not as good as a single card with 2-3GB of ram, but it is still pretty good. Most games cannot take advantage of 3GB of ram unless using 3 screens, so it is generally not a problem yet.
Each card will get a full 8 lanes (or 4GB/s) and the card controller will distribute the bandwidth to the card that needs it at any given time. However, the game is changing quickly on this. PCIe3 allows for 1GB/s/lane, which would drive a quad setup much better. Or some of the new SB-E mobos allow for a PCIe2 configuration of 16x16(x8 for a 3rd card), which would not likely choke the 590's in quadSLi. But at some point next year (no official release dates yet) will come the PCIe3 cards from nVidia and AMD which will allow for much more throughput. This is useless for people like me running a single card, or even people running 2 midrage cards, but if you want to run 4 gpus then this is something that may be worth waiting for before going all out on a build.
Lastly, before blowing all your money on 2 590's remember that they have heat issues with the 590, so a single 590 is not as fast as 2 580's. And 4 580's should handily beat 2 590s. So keep that in consideration as well. But if running that many cards then you will want to go with a SB-E, or the older high end i7s which allow for more PCIe lanes than the chips made for us normal folk.