I have one of the MSI 5 - PCI-express motherboards, 4 Gigabyte 6950's and a 1000W power supply.
The 6950s have a typical power consumption while gaming at 170W. Mining will be even much less than that. How come you can't power them through 1x 1000W power supply?
1. Is it possible to have two cards in SLI and two not so they can mine while gaming?
It's Crossfire for AMD cards, but yes, should be fine. Just remove the Crossfire cables on the 2 cards you want mining. You'll need to use a miner that lets you select the OpenCL devices to mine with.
2. I have a cheap 500W Antec power supply laying around. Could I use that to power 1 or 2 of the 6950's to take the load of the main PSU? I've seen dual psu-adapters online, however I get mixed messages on whether it's recommended on doing that or not.
The dual-PSU adapters AFAIK just connector together the green wire on the ATX connector to power up the 2nd PSU while you turn on the first one normally. Should be perfectly safe.
3. Newegg has an Antec 1250 PSU for ~$150($50 off practically), only 6 pci-e connectors though. If I bought it to solve my power issue, would it be ok to use the molex-to-pci-e connectors? The one's that came with the video cards are cheaply manufactured. The connectors in the molex are usually going all different directions and can be difficult to connect.
Antec doesn't make a 1250W PSU. They make the TPQ1200 and the HCP1200, but neither are $150.