Taken from: http://forums.caleague.com/showthread.php?t=91980

Launch Options

Source has many performance-friendly launch options built-in that you may find give you a significant performance gain.

Launch properties can be added by right-clicking Counter-Strike: Source in Steam's My Games menu, clicking Properties, then Launch Options.

-dxlevel: Sets the DirectX level Source runs in.

You may find that setting this value lower to than what your card can actually support yields a performance increase. For example, a DirectX 9 card may run significantly faster in DirectX 8.1. Again, it is best to test various settings to find out what is right for you. Use the following:

    * -dxlevel 95 for DirectX 9c
    * -dxlevel 90 for DirectX 9
    * -dxlevel 81 for DirectX 8.1


-heapsize: Dedicates a specific amount of memory to Source.

Increasing the allotted memory can yield substantial performance gains. However, it is important that you dedicate the right amount of memory. Dedicating too much or too little could actually cause a performance decrease.

It is recommended you dedicate half of your system memory. Below is the recommended usage:

    * 512MB System Memory: -heapsize 262144
    * 1GB System Memory: -heapsize 524288
    * 2GB System Memory: -heapsize 1048576
