Believe it or not, the GM Service manual does not list the wattages for these bulbs. I know that you can change the 880'* (35W) to 883'* (?)(50W) and then change the fuse to a 20 amp from the 15 that'* in stock. I would suggest that you go to an autoparts store and look at the replacement packaging for that info. My best educated guess would be that you should mulitply the stock wattage by 43% to get your new high end. That'* the difference between the 35 and 50 watt fogs.
As an example of wattage/current: 35 w/12v=2.9 amps
The difference ends up being approx. 1.2 amps more load when upgrading from 35 w to 50w lamps. I'm not sure if you really have to go up to the next size fuse on the supply side of that. Hope this helps a little.