When the new PowerMacs are announced at WWDC in June, I wouldn't be surprised if they are quad core all around. But they will have to be based on Conroe for 64-bit Extensions to the x86 architecture. Actual shipping dates would be dependent on Intel's release of said processor, and it may take a few weeks to a couple of months after WWDC. But if Apple has lit a fire under Intel, maybe the wait won't be so long.
As for SLI, I cannot see Apple supporting a non-standard. If they support SLI, the ATI camp will complain about the lack of CrossFire.