Actually, I wouldn't object so much to a Xeon if it meant even another $100-150 extra on the cpu+mobo together, but having 0 experience with that group of hardware I'm at a loss to say exactly why I or anyone would go that specific route. Is the Xeon bringing anything to the table that a discerning but not overly specific user might gain from? I've been working under the assumption that these "server boards" necessarily come with > 4 ram slots, or tricked out controllers; and some type of super-RAIDing capacity, basically a huge supply of something which I don't demand.
x86 workstation stuff isn't that much different from regular desktop x86 stuff. There is less variability and they are supported longer. With Intel, you're getting all the bells and whistles they cut out for segmentation and binning. With AMD, you're getting guarantees about support and availability of the chip since their desktop procs already have the features enabled.
Workstation/server boards generally come with less. They don't have alot of fancy stuff, but they will have stuff like quad NICs, COM ports, and IPMI with KVM over IP. They are the epitome of stable since the people buying them are expecting them to run 24x7 for many years, and they also expect to have parts available to fix them over those years. Consumer stuff is disposable by contrast. This, of course, means the workstation/server stuff can get a little stale, but it's for other people to beta test the new hottness.
More then four DIMM slots is dependent on the board. Most workstation ATX boards only have four slots.
RAID is dependent on the board as well. Some boards have RAID chips, and others make do with the SATA lanes on the chipset.