The 2 you have picked out are the 6600GT and the 6600, respectively. For Nvidia cards, they have several cards with the same number (ie:6600). But, they make a few different versions of that card to suit people's budgets and performance needs.
The 6600GT is the better one of those two cards.
Have a look at www.tomshardware.com
and look for the "VGA Charts." They will show you how the cards stack up against each other, and against an FX5200.