Couple of extra clarifications:
- Gaming on one or two monitors ?
- What programs for 3D design ?
- How important is the real time rendering speed in the 3D design?
- Do you actually work with 3D design as in does it pay the bills ?
-Two 24" monitors, although I tend to use one monitor to keep a browser open.
-I'm using HeroEngine + Maya (HeroBlade/HeroScript), it works on my current system well - but a bit laggy. It doesn't pay the bills, just a hobby. It's not super important, but smoother is always better. =)
If the 3D design is just a hobby for you, a 1155 based i7 2600k build will work better for you and let you get more gaming juice.
But here is one way to spend 3k USD on a computer that should be great for both gaming and the 3D modeling.
Basically changing the CPU and Mobo and using the saved money to another GTX580 would make the more gaming oriented build.