I need your help everyone....

My PC specs are:
**Intel dual core E5300 2.6ghz ( didn't overclock )
**Nvidia GeForce GTS450 (stock clock is 783mhz, I down clocked it to 740mhz because I'm scared it will hard lock during battlefield like some people do with their card, please correct me if I'm wrong so I can revert it back to normal )
**3GB ram
**1280x720 monitor refresh rate 60ghz.
**Windows 7 professional 32bit

I play battlefield multiplayer with 15-55, average 30 fps with EVERYTHING set to low. Except for my resolution is set at 1280x720. Fps drop below 25 during heavy battle.

I need to know are there any other tweaks to improve my fps. Currently using gamebooster to help. And no, I don't want to buy a any new card or CPU for now. So what else can I do? Overclock my gpu? Overclock my CPU? Is it safe?

Please input any suggestion to increase my fps without buying anything new. Thanks in advance for those who's helping!