My favorite screenshot from my 3D game runs at 20 fps and this is with bad design. If I optimized it, I could easily exceed 100 fps, even with nearly 2400 buildings and the whole level in view. This was with Windows 98 and with XP Pro now, it's 50% faster adding another huge impact. With the use of LOD, my experiment results, and other tricks, I can make a level spanning 200,000 quants in diameter and still get a monster frame rate. I'm waiting for bug fixes (especially the crash in MED when passing 16 groups) before I resume my 3D game. My 2D game is nearly done so that's my top priority.

The best way to optimize is to learn optimization tricks and, most importantly, run experiments. There's a thread in the engine forum about performance tips and tricks, of which I posted a graph showing the effect on performance.