The only thing you can volt mod with pencil on the 7800gt is the mem voltage. You can mod your bios to provide up to 1.5 volts, but if you want past there you have to hard volt mod it.
Edit: here is the link for the hard volt mods:
Link
and here is the link for the core bios volt mod:
Link
If you don't want to solder on your board then do the pencil volt mod for memory and the bios volt mod for the core. Other wise get some variable resistors at radio shack solder them in there and have some fun. Be sure to get a volt meter and some better cooling for the 7800gt.
Good luck. Share your results with us when your done. I can get about 560 on the core and 1330 on the mem but it is only stable enough to run bench marks I usually run it at about 550 and 1300.