I dont think dpi has anything to do with it.
It is just how sensitive it responds to movements. So a higher dpi would mean being able to respond to the slightest change, and respond quicker than a lower dpi.
The sensitivity in the game is about how far your mouse move vs. how far the aimer on the screen moves.
High sens. would mean your mouse moves abit, the crosshair can move alot.
And low sens... you know...
High sens in cs is good if you need to look back and clear an area, so that you can quickly check an area without moving your mouse across the table.
Low sens is good for aiming bascically. makes your more accurate.
Depending on your computer settings to influence on your game settings, You could try to start with 5 and adjust thereafter.
For me i use 4.5
