A few tips from experience:
1. Try adjusting your mouse sensitivity. I adjusted mine from the default to 9.7 with my old 9500 gt. With the HD5770 I have now I decreased it to 7.0.
2. Your video card is good to run this game at decent settings, however maybe try playing around with resolution settings and MSAA/AF settings. I find that even if I can't get the game to run in full eyedcandy glory (9500 gt), kicking the settings down to get better frames = better reaction times = more points per round.
3. Like others have made mention, work on joining squads and playing a support role to your team/squad. Also i recommend using an engineer simply because you can take out vehicles if you happen to run into one on foot. (if the driver/gunner is inept that is)
4. Along with joining a squad, stick with your squad, you can always spawn on them once you die. Not only does this give you a better chance of survival but it teaches you how to be a team player. If your squad isn't sticking together take the initiative and politely remind them to stick together and that they will be more effective if you work together.
5. Don't get frustrated, I find the more calm I am at playing a game the better I perform. (Look me up on combat arms for pistols, ranked less than 100 in the USP SE, USP TACT SE, and K5SD my name on there is Albeno)
6. Try out assault as well, the underbarrel grenade launcher (AKA Noob tube) is a great weapon and easy to master quickly. Plus it helps get those pesky snipers out of buildings.
Hope some of this helps your phlight. Good luck and above all else, remain steady!