Not sure if you still want advice, but I'll give you my two cents as a fellow Mac owner.
I own two Macs, a Macbook Pro (2008) and a Mac Pro (2006). They're a little dated, but I bought them used. The Macbook Pro uses a GeForce 9600 GT, and I basically use it for mid-range stuff, nothing too serious. Eventually, I needed something that had as much horsepower as a gaming PC. So I looked on craigslist and found a guy selling a problematic MacPro 1,1 for $200. All it really needed was a new HDD as well as a new video card because it was dying. Bought a new HDD for $10, and bought an OEM version of a GeForce GTX 760 for $175. Follow up with a workaround to install OSX Mavericks, and viola, an inexpensive Mac running 10.9.3 w/ OpenGL 4.1 support.
Even thought the MacPro 1,1 is 8 years old, it's actually much more upgradable than some people claim it is. With the later versions os OSX, you can install a PC video card which works like normal (in most cases; and minus the boot screen), but it has to be an NVIDIA GeForce 8xxx or better card. AMD cards don't work out of the box last I checked. If you need a decent amount of RAM, even the MacPro 1,1 supports up to 32gb of RAM at a minimum (later models will support more). If you need more CPU power, you can upgrade the CPU from 2 Xeon dual cores to 2 Xeon quad cores (up to 3.0Ghz). There's also some relatively cheap Airport (wifi) and Blutooth upgrades that attach directly to the mobo, keeping your PCI-e slots free. I spend a total of $405 so far, I just didn't upgrade the ram and CPU yet.
If you can afford a 2010 model, you can go for that. Just be sure you know what you're doing if you're going to be a cheapskate like me.