I'd recommend Call of Duty 3, and then rent Gears of War.
While GOW is an amazing looking game and quite fun, because of the game design replay value is fairly small unless you're a gamer who
has to have all of the achiements from a game.
Multiplayer doesn't hold a candle to a good few rounds of Unreal Championship 2 or Halo 2... which you can pick both up at Game for a £10.
CoD3 has a far more in-depth story. Looks great and will take a while to complete; GOW you can do in a night.
Intel Core 2 Duo E6400, 512MB DDR2 667MHz, ATi Radeon X1900 XT 256MB PCI-E, Windows Vista Business / XP Professional SP2