Woah ...
First thing, neither of these cards are good buys. The 6600GT was replaced by the 6800XT and now replaced by the 7600GT. The X850XT was replaced by the X1800XT and now the X1900XT(X). Next, the 6600 series is mid-ranged, while the X850 series is higher end. Nevertheless, they're both outdated and pointless to buy.
If your looking to buy a video card around $200 here are your best bets:
7600GT (Nvidia)
X1800GTO (ATI)
The X1800GTO should be faster than the 7600GT but costs an extra $25 or so.