the 8400gs will crash on most games as its a video card and in real terms below recommended spec for most games of the last 4 years. the hd 2000 is more stable but not much stronger but is entry level for some games while the gs defiantly isnt. do not pay any money for it as its worthless not even worth the £18 dabs are selling them for.
hd 2000 will give minimum settings and rez on a game like ac2 but wont play bf3
save up 100-150 and you can then play games at medium at the minimum. (as long as you have a strong enough psu)