I wouldn't worry about disconnecting the battery... Ive changed seats out in my 01 S at least 4 times with only disconnected the seatbelt switch... basically as you will see in the instructions, there is a switch that connects to the belt connector that activates the airbag... if you insert the key without the switch connected you will get an light... thats all
here are some instructions...
http://gallery.rennlist.com/gallery/albuq41
http://members.aol.com/ericbosco/gt3/page_01.htm
http://quattro.me.uiuc.edu/~jon/Porsche996/GT3_Seats+Rollbar/
One of the trickiest parts is getting the factory seats off the rails... I found the best method to be to go at the seat from the side by tacking off the side plastic trim pieces...