Surprised you have managed to get both working at once. Windows just loves to reject the mix of the two companies. Or maybe it's just something built into the driver
installers themselves. (it wouldn't surprise me)
Anyhow your graphics should be capable of playing just about anything on the market, though you might have to drop settings in some of the newest stuff.
The above mentioned is a decent game somewhat like counterstrike.
FEAR combat is also still free to play, it doesn't include the single player game though.
Lord of the rings online can be played free, but honestly after about level 25ish, you'll start running out of active quest content without purchasing a pack. It is possible to max a character without doing so though.
There is ACE online which is kind of like a sci fi space fighter kind of thing.
alien swarm is available free on steam (as are a multitude of games both from known companies and indie)
America's Army 3 is also free to play, it's very involved for a shooter though it's not really something you can just jump into.
Battlestar gallatica online is free to play as well
Crossfire (another counter strike like game)
Dark Orbit I believe is still free to play and they offer prizes of real money for their monthly battle winners.
actually just have a look here - All Games - Free Multiplayer Online Games
the list is real not spam or bs, they tell you if it's trial or if full services require paying.