What is the make and model of your PSU?, i play battleifled 2 on highest settings with 3700+ AMD @ 2.8ghz and ATI RADEON X800XT PE, and i only get freezing now and again, and you graphics card is a hell of alot more powerfull than mine,i also never get low FPS in most games on high settigns too, so it seems like it could be your powersupply, i only used 1gb of ram when playing battlefield 2 aswell, either its your power supply, or ram, but seems to be your powersupply, unless you've got some faulty hardware.
Have you plugged in all power cabled into the Graphics card?
2gb of ram for battlefield 2 and other games like F.E.A.R is reccomended though.
the 7900GTX is one of the best cards out at the moment, if not the best!
your cpu is a socket 775 right?, well you can have DDR 2 memory i think, but i think if your going to buy more memory, stick with performance DDR 400 memory (PC 3200), but thats costly, so just upgrade to 2gb of memory (buy the same ram as you have now), also, do you have Dual Channel enabled? (only applies if you have 2 or 4 sticks of RAM)
Your CPU seems ok, RAM ok, and graphics card BRILL!!!, seems like your power supply aint giving you enought power, the wattage aint everything!, its how many amps on the 12V rail that counts, 28A-32A is BRILL!!, but dual 12V rails in a PSU is better.
Place your computer specs in either your signature, or my computer (located on dark blue NAV bar at top)
provide more details aswell, E.G, RAM: type of ram, eg DDR 400 (PC 3200) power supply : Make and Model.
If you dont kknow much details about your computer, you can download a program for free called EVEREST, or CPU-Z which tells you alot about your computer, CPU-Z tells you alsorts about your CPU and Memory.