The Xeons are optimized for server work. I would want to see it benchmarked with the actual programs to say which is better suited for rendering. I would not be surprised if the new Opterons actually turn out to be best for rendering - the previous crop would usually beat the previous crop of Xeon at anything to do with huge amounts of data. Intel may have leapfrogged AMD in raw computing, but I think AMD still has the edge in data throughput until Intel manage to make a processor with onboard memory controller. You will need lots of memory, and it is nice to have a system which does not force memory access and disk transfers through the same bottleneck.
In any case, I would recommend to look into a water cooled system - it is more efficient, more reliable, and a lot quieter. For this kind of system, the extra cost is not all that significant.
OP, supposedly the next generation of processors for both AMD and Intel are meant to be more energy efficient and therefore, probably cooler running. Are you suggesting the water cooling because of the increased number of processors or is that based on current technology? I have run an old XEON workstation with twin 2.0 Ghz processors, and when I added another GB worth of RIMMS to the GB I already had, after some rendering the machine would shut doun to self-protect from excess heat. By making the fan run high speed as default, I never had the shutdoun problem again, although my ears still hear phantom white noise to this day.
So, I'm wondering if the water cooling you suggest is for performance push, or for equipment protection?