2d and 3d?

[Streamline]

Me and my friend currently just built a new a64 3200+ w/ the evga 6800gt and we are looking into OC the gt. We are using the x-setup pro. we installed the plug in for the oc option in the advanced menu for the graphics card. In the clock freq. option there is 2 options 2d (standard) and 3d(performace) what is the differnce between these to. which option should i use and OC? or should i set both the same frequence. Thanks