Getting My first pc game tomorrow :D

Hi my name is sam and i was a xbox 360 gamer till i got my new graphic card (AMD Radeon HD 6870) so now im more stucking on my pc because i want to know wich game i should buy tomorrow?I usually get bored very easily on a game so i tought of getting left 4 dead 2 because there is alot of custom campaigns that player make so you don't get bored with the same one over and over...I also tought of getting battlefield bad company 2 becaue i love playing a game with amazing graphics but sometimes im hesitating because this game require some good computer specs and im scared of getting bad FPS and also battlefield 3 is coming this years.

So what do you guys think wich game should i get and if you want to you can propose differents ones.

Thanks and By theway im really sorry for my bad english xD

My comp specs:Phenom(tm) II X4 820 processor 2.80Ghz (i still can OC it but i don't wanna take the risk)
6 GB of RAM
1 TB of disk space
Direct X11
AMD Radeon HD 6870
And by the way when i mean ''My first pc game'' i mean a game that you buy in a store in a box with a CD DVD ROM or something like that

Bad Company 2, GTA IV, or L4d2

MINECRAFT

No, don't listen to them peoples. Counter Strike: Source is the game every PC steamer has.
Get Steam, it's like PSN/Xbox Live on the PC.

Crossfire, Combat Arms, MAT theese are all free