On the CPU side I would say that you're good in the middle to low-end range. I think the source of your issue(s) is your ancient/non-openGL 3.2 conformant GPU. OBS uses the GPU for compositing, so if that part isn't up to spec, you'll have issues, esp. on the *nix OSes.
As to OpenGL 3.3/3.2 vs 2.1... I don't have a non-technical way to explain it, but basically in Apple land they supply a base driver/harness that supports OpenGL 3.3, and they write some work-around code/Soft emulation code for certain card "levels".
What this means is that if you hardware doesn't support 3.2, Apple has some fallbacks in place to keep the OS running, but if you try to run something that actually relies on having that Hardware there you'll have weird issues, random bugs, or it just won't work.
For the record I have a Dual Xeon x5650 setup, and can record at 1920x1080@60p@medium and only use from 2% to about 40% per core when encoding at those specs (overall CPU usage is at 26%-40%). Your CPUs are a generation or 2 older and you have 33% fewer cores then me, but you should get good performance out of the cores. How much? I can't say without doing some testing.
