wow, there´re 6600 cards out there with 515mb. didn´t know that...
assuming you´re familar with driver installation/deinstallation try ver 66.93 of the nvidia videocard driver. It´s maybe a better choice for your card.
http://download.nvidia.com/Windows/6...ernational.exe
what about mods? are you using any (esp. visual improvements) you havn´t used before? F.e. the increased visibilility mod is a big framerate killer (but i´d not play a single minute anymore without it ;-) )