Check to make sure you actually use your 1 gig of RAM, if you can disable some apps, then do so.
Still, 2 GB is very popular now, and is almost necessary for BF2.
After that, upgrade the video card, still that's a lot of money....
You probably paid, what, $340 for that 6800 GT, and now to upgrade you'd have to pay around $300 for a 7800GT?
Also, if you haven't already, overclocking is an option.