Well I'm running an Intel Quad Core 2.5 gig processor, 6gb ram, with an Nvidia Geforce GTX 660, operating windows 7 ultimate. I'm not experiencing any issues with my graphics that I've noticed though I just became curious about it when I got that message.
AGP is actually the port-type of the Videocard.
Since you are using a GTX 660, that is a PCI-E card. No AGP port on it. Perhaps you can emulate the game to "think" it is using an AGP, but for that you would not an appropriate executor file.
I do not know where you obtained the game, but if you did get it from GOG, then usually it is usually a game that was allready optimised to run on various OS-es, and various videocards.
So sadly there is no way to change the settings to run it as an AGP card. However i am sure that there are options to increase the games experience.(Unless you allready reached the max, which i cannot state not knowing the game, since they forgot to
give me a code to it when i bought it on cd:P)