I like your idea of building a good workstation and a couple (quartet?) powerful less expensive systems for a basic render farm.
I can't see how 12 Xeon cores would outperform 16 i7-950 cores.
And you'd be able to have a workstation system while the render farm is busy.
Would a dual Xeon quad workstation fit anywhere into the budget picture?
How flexible are you on the timing of putting this altogether? Those new Sandy Bridge CPUs due out (again) in a few more weeks give you more options.