NV42 is only used on PCI-E 6800s which have the 4th pixel pipeline quad laser cut and thus impossible to unlock, 6800s AGP still come with the NV40 core and still have the 4th quad disabled meaning you can try to unlock in any 6800 AGP but not in a 6800 PCI-E.
And I would just get a 6600GT from the store, I mean after all, that is just your backup gaming rig, the 6800 offers variable 1fps (hl2) to 5-10-15fps (most games) to 30fps (ut2004) advantage over the 6600GT but if its more comfortable to just walk to the store and buy it, then by all means do it, once games in that rig start to become unplayable, chances are they would still be unplayable with a 6800 (say you get 10fps on the 6600GT and 15fps on the 6800, thats unplayable for both cards) and you have your main rig anyways.
Summary: Get the 6600GT.