Good shooter for an older PC?

Depend on what you prefer: single player or multiplayer.

For example I still could play Return to Castle Wolfenstein (even it's very old game).

Other games:
Medal of Honor: Frontline
S.T.A.L.K.E.R.: Shadow of Chernobyl
Borderlands 2
Arma 2
BioShock
Left 4 Dead 2
Rainbow Six Siege