Operation Flashpoint Dragon Rising system requirements:
# 7900 GT / 256 MB VRAM or higher)
# Sound Card: DirectX Compatible
# Memory: 1 GB
# Hard disk: 8 GB
# Dual Core CPU 2 x 2.4 GHz
PSU = Power Supply Unit
Keeping in view the above requirements, your current system will struggle to cope with these requirements even with the new graphics card. The from selection of graphics card point of view, I wouldn't bother looking at the lower end of the cards then, perhaps 3850 or 4670 are realistic choices. Also whichever card you choose do check its PSU requirements.
To play LFD2 you will require 2Gb you have 1744Mb less than required
To play Operation Flashpoint Dragon Rising you will require 1Gb you have 744Mb less than required
I have a NvIdea Geforce 7600 GT (512Mb) bought recently $120AUS and only 1553Mb of RAM so I'm unable at this stage to play LFD2 too (I only have a CRT monitor).
#9 in this thread has already taken care of that issue; but thanks again ucloud
And emman it doesn't matter how much upgradation you do to your current system; it will not take you much further with regard to playing games like L4D2.
So, if you want casual gaming just get one card from the choices I've gathered for you in your system; and thats about all I would recommend, keeping in view all the facts. Spending much more on such an old rig will not make sense in my view. Importantly I doubt very much that you'll be able to find 'new' ram (DDR266 SDRAM) modules for your system anyway; and even if you do, they will be exorbitantly expensive, again an unwise way to spend your money.
You can either ask one of your local hardware sellers (who offers you a good deal ofcourse ) to get one for you; or you may order it on the web, though it will take some time to get to you in either case.