I'd say up the ram.I have an x300 6000 with 1 GB of ram and play Call of Duty and WoW with no problems...both are more demanding than CW:ET.I'd say get atleast a stick of 512 to put in with your 256.
Do a search for omega drivers download the latest version and theres a tab int hem that lets u overclock them its as easy as sliding a bar up and down.
Use the drivers that came on the bf2 cd, if they are not on there then they are included with the demo for sure.Its a much higher version than what nvidia has on there website.