Hi, I tried running Left 4 Dead with the following key components:

Intel E6300 @ 2.8GHZ
Kingston 2gb DDR2 667mhz
Geforce 8400GS 512MB 450mhz
Cooler Master 500WATTS PS

However, I have very unstable frame rates (yes I have installed the latest drivers for my mobo and the gpu) even when I choose the video settings to very low. I have tried running on Can You Run It? and it passed the recommended requirements easily. I have tried the game on my previous computer with the following key components:

Intel E8400 @ 3.0GHZ
Kingston 2gb DDR2 667mhz
Geforce 8800GT 512MB 640mhz
Antec Earthwatts 500WATTS

With the above parts, I could run the game smoothly even at the highest settings. The only difference I see in these 2 computers are the GPU and the processor in which the first com isn't very far behind...so pros, please help me on this! Thanks.

Both cards arent great performers, L4D isnt an old game, I'd guess you may be getting what 30 fps, maybe?

I don't think this is a problem, i'm pretty sure that is typical 8400gs performance.

With my setup , there is some uneven framerate issues, however use the 175.16 drivers , and i can play at 1280x1024 high to very high, keep effects at low with AAX2 and get between 15-30fps. If you want fraps between 30-50, you may need to buy a better card, the 8400gs is ok, but nothing great.

You may think the components are close in comparison but the video cards, in particular, are quite different. I mean, there is a big performance gap between the 8400GS and the 8800GT. One is used for basic video apps, and one is for gaming. Simple as that.

his hardware is completely different from your, his card offers 10X the bandwidth and is more than likely running at a higher resolution.

I see...thanks for the comments!
Ok so there is a big difference between those 2 cards. Got it. The 8800GT is not a great performer, but it runs my Fallout 3 and Crysis pretty smoothly. Oh i did not mention that the 8400GS was running at 800 X 600 which is the lowest resolution and that the 8800GT was running at 1280x1024 with a constant almost 60fps as compared to 8400GS's 30fps (and sometimes even lower than that). Anyway now i understand that cards with the same dedicated memory doesnt mean same performance. Thanks guys for the great comments.

i dont think a 8400GT cannot play a L4D at a decent 30fps over a low L4D settings.
i used to play a L4D in Pentium M 1.70 and ati mr 9700 64MB, it can play L4D at low settings 1024x768 and gives 22-30 fps with antivirus turned off etc.

while your 8400GS system is considerably much powerful than my old laptop. i believe it can do better. perhaps Dox driver?