Valve has done such as good job with making L4D2 one of the most popular cooperative survival shooters out there. Valve also consolidated the maps from the original L4D and added several more on top of that. The replayability is excellent.

L4D2 is typically a 4 player coop game. The game allows players to jump in and out as desired but some players prefer others to stay in the game until it's done.

L4D2 also supports survival and player vs player among other permutations.

This makes the game excellent value with easy system requirements. Even a 10 year old graphics card or recent integrated graphics is all you need. The 6600 GT has 8 pixel shaders, 3 vertex shaders, 8 TMUs and 4 ROPs which is very undemanding by modern standards.