Well I am planning to upgrade mine over the next few months as this one is getting pretty old now but I'll provide the specs as best I can. I also use this computer for games as well as digital art. It is not a particular make though - it was a barebones system that I upgraded,
i7, 8gb RAM, GeForce 750Ti. Running Windows 7 Professional. Generally work between 1500x1500 to 3000x3000 - sometimes more depending on the thing. I still use Photoshop CS3 though which seems to hate larger canvas sizes (I think it can't make use of my graphic card's memory), coupled with 8gb RAM which seems low these days. SAI doesn't seem too bothered though. This system still runs most modern games fine though. I can't tell you exactly when I purchased it though because I forgot.
And I use two monitors.