The minimum requirements for L4D2 are:
Intel Processor - Core 2 Duo E4600 2.4GHz
AMD Processor - Athlon 64 X2 Dual Core 4200+
Nvidia Graphics Card - Geforce 7600 GT 256MB
ATI Graphics Card - Radeon X1600
RAM Memory - 2 GB
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.