Well, there's absolutely NO risk whatsoever to you or your computer.
Here's a video that explains very well what bottlenecking is : http://www.youtube.com/watch?v=TGdo75gasaQ
It's not going to slow down your computer.
Your graphics card should handle it at medium...barely. You might get a 15-20 fps. You have to understand, it's a fairly old card (from january 2009) and MW2 is kind of recent (less than 1 year, I think). If you still want this card, I'd go with this one : http://www.diamondmm.com/4650PE1GHP.php.
Hope this helps !