How much does TA experience impact acceptance into PhD programs?
From my perspective admitting students to CS PhD programs:

TAing for CS classes will be a small positive signal that you did well in those classes and were well thought of by the instructor (who presumably asked you to be his TA, or at least accepted your application). It might also let you get to know the professor teaching the class better, which will be helpful for your application if he/she can write you a letter of recommendation. But beyond that, it won't be a huge bonus in and of itself -- its mostly a signal correlated with other good things.