You can SLI those fine, just it both will run at the slowest card's speed. e.g. one card running @ 675Mhz core and another running @ 775Mhz core, when SLI'ed, they will both run @ 675Mhz. That applies for memory, shaders, and RAM too.
Possibly off topic, and sorry if that so, but I keep reading this yet I haven't seen it myself. I had a couple of PNY GTX 560's, one was an OC model at 850MHz the other a OC2 model at 900MHz. According to MSI Afterburner and GPU-Z they were both running at their own speeds, not synced to the slower card (and it didn't matter which one was in the primary slot either).