What PSU?
As for SLI, some games that are not SLI Optimized will not reflect the 2nd Card in action. Setting up SLI is as simple as
attaching the SLI Bridge and enabling SLI in your NCP, really thats it.
What GeForce Driver are you using?
Have you tried a "clean install" of Drivers?
What kind of 3D Mark Scores or Vantage Scores are you geting? Your Rig really should be a game killer with that Quad CPU OC'd, that RAM and SLI 260's.
Jst looking back again at your Post, I can tell you that Counter Strike Source isn't a SLI Optimized game at all, actually you proved yourself that SLI is not required for great FPS, only 1 GPU is necessary.
In NCP, simply select "Use the Advanced 3D Image Settings" then in "Manage 3D Image Settings" ensure Ambient Occlusion is "off", shouldbe by default. You souldn't have to change any of thiese settings.
Good luck
