I've found that you can disable shadows and gain a lot of performance with very little noticeable graphical change.

As for the specs, if he's not going to upgrade the system, why do they matter? The things that help performance do so generally no matter what system you are running, so I fail to see how his hardware is at all related to the topic of this thread. If he was going to upgrade then it would be helpful so we could suggest what was most in need of upgrade, but as he's not it seems entirely moot to me. To run with your analogy Forward Observer, it's less like asking what gas mileage you'll get w/o knowing the car - more like asking how to get better gas mileage without knowing the car, which is in fact substantially different and very answerable, where the other would not be.