The Opterons them self should overclock as other AMD64 cpu's, the problem is finding a motherboard with oc'ing options i the bios, and ecc/reg ddr which can handle high fsb, that won't be cheap, probably cheaper to by the faster cpu.
You should wait for the Iwill DK8EW, because the DK8ES doesn't support SLI properly. It only dos 16/2, not 8/8 lanes as the official SLI requires. The DK8ES will not come with the SLI bridge, the DK8EW will. (the W in the end stands for Workstation, S stands for server)
The DK8EW is...
What is the PSU requirements on the SLI setup? Remembering the 460w recommending on the 6800ultra I wonder what you need to run at 6800u sli and a FX55 cpu system?
How mutch does the cpu maters in the games as Doom3, Far Cry and Half Life2, if you only have a 3000+ cpu, will buying a SLI...