4 socket vs 2 socket
I disagree your statement that a 4 socket x86 server costs $100k. I just checked both dell.com and hp.com and could configure a 4 socket (6-cores per CPU) server with various configurations ranging from 16GB to 128GB. The cost of HW plus basic warranty was between $13k to $22k.
I do agree with your comment that 2 socket servers may be sufficient for most jobs. This has been the case for many years and will likely be for many to come and is validated by any of the major analyst reports, IDC, Gartner, Mercury. These servers offer sufficient performance for the majority of workloads that exist, vitrualized or not virtualized, and offer exceptional performance per $ and performance per Watt.
However, a 4 socket server has it's place as a purpose built server and are often deployed by IT for a certain class of business applications (database, ERP, svr consolidation) where more compute, memory, I/O and reliability are required to handle the task. I would never buy a Hummer on MPG. I don't think this fundamentally changes in 2009 or beyond.
Here is an example (http://www.intel.com/references/pdfs/Turtle_Entertainment_V3.pdf) where by maximizing their consolidation with a 4S server an IT customer saw a 30+% reduction in TCO while improving their customer's gaming experience.