Most folks don't even need 2, let alone 4 PCIe x16 slots. The best single-slot card is only a hair slower than the best 2-card setup. Your returns start diminishing quickly, and you pay more and more money. You could spend $2,000 on 4 of the BEST PCIe cards, and still only perform 20% better than any one of those cards running by itself.
It's like selling a car, and claiming one of your best features is 20 cup holders placed throughout the cabin. Well most folks only have X amount of people in the car at one time, and only need X amount of cup holders. Same goes for video cards and multiple slots. My $0.02 on the matter.
As for the Intel/AMD 4-core situation, having all the cores on 1 chip has a benefit and advantage, in that they communicate with each other faster. As has been mentioned nobody's going to benefit from having 4 cores any time soon, but if this is based on the recent Conroe architecture, and AMDs is based on older chips (used together), then the better of the two will most likely be the Intel chip. It will overclock far better, with far less heat and power consumption (based upon Conroe performance). Don't get me wrong, I have respect for both AMD and Intel lines of chips, but Intel has clearly pulled a rabbit out of the hat, and will continue to do so for a while until AMD catches up. AMD lumping 2 of their older chips together is a stop-gap marketing method (again, nobody needs 4 cores right now, regardless of who makes 'em!). If they actually redesign a chip or make a new chip, which could take years if they started now, then they'd give Intel some competition.
For now, Intel's got AMD on the ropes, including the 4-core arena. That's my take on the matter.