Where can I find print or online information on the archmages and/or the War of the Archmages?
Though the magical battle between members of the Circle of Eight on the day of the signing of the Great Treaty in Greyhawk City is mentioned in passing in the From the Ashes boxed set, it's the 2e adventure Return of the Eight that has most of the information on it and on the mages, including full stats on Jallarzi, Tenser, Warnes Starcoat, and Theodain Eriason. Then the first issue of the Living Greyhawk Journal (LGJ #0) has an article called "Wheels within Wheels" that details the entire history of the Citadel of Eight and Circle of Eight including 3e stats for all of them. In fact, this link from @yhw42's comment in another answer is a transcription of that entire article minus the stats.
Start with Wikipedia, which will give you some info on the Circle of Eight. Use the footnotes for further research.
I'm not too familiar with the term "War of the Archmages". The mages of the Circle of Eight fought during the Greyhawk Wars. The definitive resource for that is the Greyhawk Wars boxed set. It is fairly expensive on Ebay. Unfortunately I believe my copy is long gone so I can't give too many specifics.
\$\begingroup\$ @yhw42 - yeah, I was looking at that. Unsure of where he sourced that from. Other pages at the site suggest that the info includes material from his home campaign. I may not be looking deep enough though. \$\endgroup\$ Jan 24, 2011 at 20:54
\$\begingroup\$ Oh! I missed the part about it being his home campaign. That history seemed to line up with the others I found, but I couldn't find any sources/citations... It made for a prettier read, though. \$\endgroup\$– yhw42Jan 24, 2011 at 21:03
\$\begingroup\$ No, that's all canon as best as I can tell. \$\endgroup\$– mxyzplkJan 25, 2011 at 1:39