Well, for your budget you won't be able to approach dual 6-core xeons. Bulldozer has 8 cores, 8 threads (4 modules), and a 3930k has 6 cores, 12 threads. If you look below I have a 3930k because I wanted to do a little something something with my adobe suite (master collection).
I can say pretty easily, this processor which is not overclocked at the moment, is amazing. Then again I went from a Core 2 Quad Q9450. I paid about $3,000 for this build, so it turned out the way I wanted for gaming and working. Sure there are some good deals on newegg.com as well.
To rival the mac pro you would need a dual processor system for starters, no single CPU is going to be able to keep up with 2 6 core Xeon's even if they are based on the first generation of the core architecture.
The rest is spent on SSD, case, cooling fans, and a video card so you can see what you are doing. It is possible to do it for less than the mac pro, but it is still very expensive, but a fair bit less than the mac if you want to do it yourself.
If it were up to me, i would probably just get the mac pro, they are beasts. One thing i would do though is get at least 16 or 32 GB in it, at least the 2008 era mac pros had very high power usage memory(ECC and whatnot) in a very cramped space so they had special massive heatsinks on them which makes a memory upgrade wayyyy more expensive than you expect so get it early so you don't have to drop a bunch extra on it later when it becomes a specialty part.
contrary to popular belief, dual xeon macpro is one of the cheapest 12c/24t workstation out there. the only negative side from the mac pro is their consumer grade graphic card (which isn't designed for professional use). i have yet to find other 12 core xeon-based system that cost below $4k (maybe someone can point it out for me )
however i wouldnt recommend a macpro for now as its current hardware are outdated, it is still using nehalem based xeon
I know theres a new one coming out next year...but this is apple, there'll always be another one coming
There will always be "another one coming" regardless of vendor unless/until the vendor withdraws from the market since nearly every technology manufacturer has annual product refreshes or substitutions to adapt to the evolving technology ecosystem and consumer trends.