No to both questions, you have to have a chipset on the motherboard that supports SLI, the DS3 doesn't have a chipset that supports it.
Why would you want a LAN card for it? It already has a Gigabit 10/100/1000 onboard port.
If you ever want to go down the SLI route, buy a EVGA 680i motherboard. Yes, it has the same overclockability as the S3 and DS3