What’s the clock rate for your CPU? You should be able to run the engine well enough to use it for lighter work, but it will chug if your levels become complex. Your GPU is pretty close to the 6850, which is only about 7% faster, the CPU is the more limiting factor. You can cancel your subscription at any time and keep using the version of the engine you have, so if I were in your shoes I would go ahead and get it, you’ll be able to use it and learn, then upgrade later if you want to have a smoother experience on higher graphics settings.
If you have one of the higher clocked Phenom II X3s it shouldn’t be that bad, the energy efficient Athlons might get to be a pain though. I find that when just working with the editor, it only stresses two of my cores (Intel i5).