what good Competitive Multiplayer to buy? any suggestions?

tribes ascend if it will run. its a tad gpu heavy

Battlefield 2 is far from dead.

Also, Counter strike source is very competitive.

Tribes Acsend, Team Fortress 2, Counter Strike, Quake, BF3 (Hardcore mode) Any COD game.

Battlefield 2 isn't dead if you know where to look.

The Counter Strike series also has a great community (Source more then 1.6 in numbers and variety, though 1.6 is a little more "hardcore")

Call of Duty series also pulls in great numbers, I imagine you could at least run CoD4, which is easily the best Call of Duty game on PC anyways.

Starcraft has a huge following for competitive multiplayer, though its an RTS and I don't know if you like RTS games or not.

There's also the Tribes series (with Tribes: Ascend being free to play, so you can check it out and see if you can run it).

There's a lot of choices dude.

super monday night combat (free)