You can find some good ones on here
http://mmogames.com/shootergamesEarlier replies we're pretty good, I'll checking out urban terror sometime also
SmokinGuns Home page Download.
Notes: Built off quake 3 arena engine an enjoyable western shooter,
Performance:Should run great on all computers, the models are done with less vertices and faces.
Warrock Home pageDownloadNotes:A decent shooter game, with air and land vehicles, motorbikes and tanks galore!
Performance:Runs pretty smooth on low, with a computer that has 2gb of ram.
Setbacks:You have to rent or lease guns, also you can't create your own character,
Each class has a different model.
To OP:You'll probably more at home with this one, because it's set up like Battlefield play4free vehicle wise...