A GPU upgrade would be a big help but you will still be hindered by the CPU. But you would need CPU, RAM, and GPU in order to get the system in good shape for the game you want. With all that being upgraded you'll probably need a new PSU as well. But even if you just upgrade the video card there is a good chance you'll have to get a new PSU as well. That being teh case you'll need about $60 to $100 for a good PSU.
Looking at the system requierments for Left 4 Dead we have:
Operating System: Windows Vista, XP or 2000
CPU: 3.0 GHz P4, Dual Core 2.0 or AMD64X2 (or higher)
RAM: 1 GB for XP / 2GB for Vista
Disc Drive: DVD-ROM Drive
Hard Drive: At least 7.5 GB of free space
Video Card: Direct X 9 compatible (must be 256 MB or more with support for pixel shader 2.0)
Sound Card: Direct X9.0c compatible
As we can see your CPU is inadiquate for the game. You should be fine on the RAM but the video card would be a big help to keeping game performance nice and smooth.
First lets focus on a good quality and afordible PSU. This allows us to focus as much as possible to the video card upgrade and CPU.
http://www.newegg.com/Product/...x?Item=N82E16817371006
I figure a video card thats about $100 leaves room for you to find a better CPU and get the PSU you'll need to support the video card upgrade. So here is a nice 9600GT.
http://www.newegg.com/Product/...x?Item=N82E16814133240
Now i assume you have a fairly recent socket 939 board. Unfortunatly the only socket 939 CPU on Newegg is the 3800. Its a 2.4Ghz CPU and should be just on the edge of supporting the game. But if you can find a socket 939 x2 or Opti that would be better. But here is that 3800 for about $40. Given the above options you will have more money availible to get your hands on a nice dual core.
http://www.newegg.com/Product/...x?Item=N82E16819103571
If you want a nicer video card then go for it. You could even get yourself a nice 8800GT or even the ATI 3850 without having to get a better PSU then that Antec Earthwatts. But you need to find yourself a dual core for your system. You may or may not need a bios update in order to get the dual core to work in your system. You may have to also add a patch to windows so it can properly use the dual core. It will use the dual core but right now it is set to use a single core. After changing the CPU out the OS may not see that it has a dual core at its disposal so it requires a certain update.