Although I never made a shooter, I did make a merchant simulation back in 2008. It ran great, had multiple towns with dynamic buildings, vegetation and a AI. It ran smooth on my 100€ hardware (ATI HD2400pro, AMD Athlon 64 2Ghz, 1 Gb RAM). And with smooth I mean 40-50 FPS.

So yeah, it is possible to have good framerate, its certainly not easy, but possible.