If any of them it is the X7800. The iMac uses the intel mobile technology normally designated the product code T7XXX. The X in X7800 signifies that this is an extreme version...
Update: The X7800 is listed as a 2.6GHz. I believe the one in the iMac is the 2.8GHz X7900. I do not think that it is on the general marked yet.
The QX6800 is a quad-core and dare I say MUCH faster processor, only to be found in the pc-guy's room. It is however running way too hot for iMac application, at least with a reasonable noise level. Think of it as a 2.66 GHz Xeon Quad core, made for single processor configuration only. This processor range is normally designated the product code Q6XXX - a 4 core derivative from the 2 core family E6XXX. A processor family sadly neglected by Apple.