The first thing that i suggest for a computer like this is to set ur 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 http://support.steampowered.com/kb_cat.php?s=68bb22f773d395ad2dda0cee50481d8e&id=66&t=qanda
-dxlevel: Sets the DirectX level Source runs in.
You may find that setting this value lower than what your card can actually support to yield a performance increase. For example, a DirectX 9 card may run significantly faster in DirectX 8. Below is the recommended usage:
* -dxlevel 90 for DirectX 9
* -dxlevel 81 for DirectX 8.1 <- Use this it is cal legal and also what i use.
The second thing that you have to take into consideration is whether or not your internet can support alot of bandwith or rates for more on rates go here: http://www.theelders.net/forum/index.php?topic=3949.0
Because of this i will put up 2 seperate configs the first will be with the rates that you should have the second will let you set ur rates freely.
Copy the following post into notepad eactly as it is.
Saving your Config
Go to File.
Change so Save as type: All Files.
Under save in go to:
Valve (if valve is not there go to steam)
Your Account Name.
Counter Strike: Source.
Name it autoexec.cfg