Halo 2 would be quite a bit of fun for the Pc.
Halo 2 would have been better if it came out in 1998 and not 2004.
Call of Duty 4: Modern Warfare > CoD: World at War.
Ask anyone who has played both. CoD4 is the best shooter (online wise) imo. If you are going for story, get Bioshock like Dhampy said.
CoD4 is the greatest of the franchise and will likely never be eclipsed.