ahzrukhalI just recently did this: I got a new mouse, with a sensor i can change to 2 times what i was use too.
I like 1inch per 360 when I play soldier, my main.
I had Windows at 7/11 mouse sens, and tf2 at 5.6 sens on a 2000dpi set mouse. This equals 1 inch per 360.
I got a new mouse. I set it to 4000dpi, lowered windows to 6/11 (default) and put tf2 at 4 sensitivity. It still equals 1 inch per 360, but my hardware (my mouse) is set to a higher definition. So what i ended up experiencing is a smoother, more responsive mouse, that has the same sensitivity as I am use to.
tldr: DPI is how well your mouse reads movement. Run your hardware as high as you can and adjust the software to maintain your desired, comfortable inches per 360. Control the "sensitivity" using tf2 and or windows.
Please take it easy on him he is new here.