GTA 4 is a very large game, even better then San Andreas, the PC version is a lot better then the 360 and PS3 version because of the higher needs for graphic.
For example YOU MUST HAVE a 8000 series, you almost HAVE to have a limit of a 1.2 Duel Core Processer, the game is very good and I never gotten tired of it.
Call of Duty 4 is very good as well. Sense I played both games, COD4 have a very short single player that can be beaten in.. maybe 4-10 hours. Very short, the game however have a great multiplayer and I was a part of Pen's rebellion toward Punkbuster and causing a 'war'.