I disagree with the low-end Xeon choice.
I would not consider Xeons getting into the picture before octa-core / higher GHz models, always on 2P boards. So we are talking something in the 2660 range or better.
The vast majority of modeling work - which for most users outweighs the time you spend rendering quite a bit - is not multi threaded.
In your case, that leaves you with 1 out of 24 threads running 2.5GHz at best, pulling all the load while the rest are idling.
A X79 board will get you all that you want in any scenario that won't require more than 64GB of ram (very very few professionals actually do), and a 3930K will be vastly faster in general tasks, while a shy slower while rendering. It can also be safely overclocked to 4.5GHz without crazy volts or cooling, making it much faster.
In Cinebench 11.5 multithraded benchmark the 2P E5-2620 might get around 13-13.5 points, while the 4.5GHz O/Ced 3930Ks play around 12.5-12.8.
A 2P 2660 16C/32T would be pushing 20s or so, and would be a nice improvement despite the single thread performance loss, but we are talking lots of monies.
So, I would think a 3930K along with a nice X79 board and 32GB of RAM (4x8GB) would do great, in most cases much better than the 2620s pair.
It cannot do ECC, but that's not that bad.
If you don't care about O/Cing, a 3960X or 3970X might be an option too - it will cost you the same as the pair of 2620s, but you get a nice factory clock boost - those are the fastest processors for 1P applications out there.
The Quadro 4000 is a nice card - no need for something more expensive if it is not for GPU rendering stuff.
In your spot, I would go for the 3930K (even without O/C) and if I would need more rendering power than that, I would spend the $500 or so saved from not getting the Xeon CPU/Mobo, to build an FX-8350 rendering node
, with future plans for extending it to a far of similar machines.