Well if you want it to play on max settings with no lag you are looking at at least 1 gig of ram 2 preferably, a new CPU, a 6800ultra or x850 xt pe gfx card assuming you have an AGP 8x slot and you will probably need to improve your cooling on the PC. But you also might hve to upgrade the motherboard, you really would be as well getting a whole new system if you want yourself an uber gaming rig, somewhere between 600 and a 100 bucks for a cheapish one.
The colonel in this post http://www.geekstogo...rts-t83688.html
is building a new system for 650 that would play BF2 on full, although some extra RAM of a better quality wouldnt hurt BF2 ideally wants 2 gigs but it does run fine on 1gig of good quality ram
Comrade general http://www.geekstogo...ion-t83795.html
is building a more expensive system at about 1150 with a much faster cpu but it would also run BF2 very well.
Those are a couple of examples from today of what you can put together.
You can improve how it will run on your system by putting another 500 to 600 into that PC if you are lucky but you are as well starting fresh components are pretty cheap at the moment from the speed of the CPU im guesin its a 478 so you would more than likely need a new mobo to upgrade it.
If you want to get into serious gaming you would be best saving up and start building a new PC for gaming.