When I reveiwed the 757 at AVSIM I posted pictures of me getting 20-ish FPS at KMIA with the 757 at "High" scenery option, medium airport traffic and 70% Traffic X.  I have a consumer grade computer from Best Buy.

Some people say frame rates are bad, but they're not.  You are just going to have pay for the unparalled visuals.  That means knowing your system, working with it, and coming up with the best compromise between performance and looks.  If you have an older system, you're probably going to have to make more compromises.  The nice thing about FSX is that it has rendered the old fixation on FPS less useful, because you can stil get very smooth performance with relatively low frame rates.