If I may throw my .02 in here...
If I had a $4000 budget for building a 3D monster machine I would certainly start with the EVGA SR-2 motherboard and dual Xeon quad cores. That will run you $2600 just for the processors and motherboard.
Having said that, I do *alot* of 3D rendering & modeling. The new Intel 2600K processor (when overclocked to 4.8-5.0) can compete right up there with the SR-2 setups. For example, take a quick look at the graph on this page: http://www.tomshardware.com/reviews/...ep,2692-6.html
You'll notice that the top two setups run this extensive rendering benchmark in 399 and 443 seconds respectively. My current setup (I7-2600K) can run that *exact* benchmark in 450 seconds.
When you consider that the top two setups use TWO processors that cost $1700 EACH, and my computers total cost was less than one of the processors and can come within 7 seconds... well you get the idea.
You should budget $1400 for a really nice 30" monitor that can do 2560x1600 (Dell U3011 comes to mind).
Now, the problem is timing. The Sandy Bridge boards are mostly unavailable right now. April will be a better time. Some will say wait for Ivy Bridge, which isn't wrong either. I suspect there will be another leap in performance when that platform hits the pipeline.
To sum up, got with a P67 motherboard and an I7-2600K if you can find one. For 3D work, the performance is simply stunning and ranks right up there with the workstation class systems at a fraction of the cost.