However, in D: OS, once I get into town or a larger battle I routinely stay at 20-30 FPS, regardless of whether or not I have the game set to ultra settings or low. Is there something I'm missing?...
Try checking your GPU utilisation with software like
MSI Afterburner and checking your CPU utilisation in Task Manager. If one (or the other) is at 100%, that's likely where the bottleneck is (most likely the GPU - I get 35-40FPS on a 580GTX though at 2560x1600).
If your GPU temperatures are low, then using MSI Afterburner to overclock it may offer slightly better performance - but do take care to ensure your cooling is good enough (i.e. try running a graphics benchmark like Unigine Heaven for 30-40 minutes).
