If you click File->"New Open CL Miner..." the program will add another tab for you to set up a worker. Then you have one worker use 0-0, and the other use 0-1. If you're not getting the same(ish) amount of hash rates from both workers, you might need to un-SLI your cards. I'm not too sure about Nvidia mining, but with ATI/AMD cards using Crossfire slows down hasing rates.

I think the main concern with quad card setups is the PSU. I'm not too sure about 6870's power usage, but you'll probably need a beefy (1000W+) power supply to support four cards under constant load. Also, the risk of investing in four cards is greater than the investment of two.

Your earnings update once per hour, and the block statistics page is delayed by an hour.

Speculation. Most of Bitcoin value is speculation at the moment. Hopefully it'll normalize and be used as intended, instead of a get rich quick mindset that it seems to be in currently.

Might need more information about what you're mining with. With the information you provided, it could be that your graphics is either overheating or the drivers are crashing (due to over/under-clocking problems).

It shouldn't cause much of a load increase, if any, since you're piratically doing the same OpenCL operations, it's just now you have two workers showing up in the GUI instead of one.

To be honest, mining with 1-5Mh/s might not be worth the electricity to run the machines. You're most likely going to get stale shares since people can run cheap(ish) graphics cards with 25x the hashing power of your processors.

Using two workers is probably a better practice since you can spot downtime/errors more quickly.

Be wary when looking for cases. In Yanz's list from Newegg is a HAF 912, which does have 8 card slots, but only two of those slots are available for PCIE from most motherboard layouts. I'd take a guess that most ATX Mid Tower are going to be the same.

How were you able to change both cards in ATI Tray Tools? I can only seem to change the card in position 0, and I have the same Sapphire 5830 as you. The cards don't have a crossfire bridge on them, which I'm assuming is what you're doing too, since crossfire doesn't help btc mining.