On a game board that I frequent (Pied Piper, if you are curious), someone asked what sort of things they should use in order to start up a Greyhawk campaign with the "original Gary's vision" feel. Here was my response; I think it illustrates some of my thinking about what a good Greyhawk campaign would use. There are a few edits.
You'll want a huge dungeon-crawl, and I would highly recommend "Castle Zayg: Upper Works" recently published by Troll Lord Games, but with the caveat that you might end up having to do a lot of design work yourself to expand the dungeon, as it is currently unclear as to whether, when, or for what game system the next module in the series will be released. I would definitely include Rob's work offered for sale here, as well; I'll definitely be placing the Living Room and Bottle City somewhere in my Castle Greyhawk when the time comes. I know some people will end up including some of Rob's Maure Castle levels to make up some of the missing levels, and that seems workable, with a little effort. You can also have fun putting in gates to Dungeonland/Beyond the Magic Mirror and the Isle of the Ape.
Adventures outside the dungeons of Greyhawk would be called for as well. You will obviously want to concentrate on the earlier material, given what you say. The original Greyhawk boxed set, definitely. Village of Hommlet/Temple of Elemental Evil. The Giants/Drow series. Lost Caverns of Tsojcanth and Forgotten Temple of Tharizdun. White Plume Mountain. Expedition to the Barrier Peaks. Not necessarily in that order... Personally, I would tend to ignore the books that TSR/WotC published on various kingdoms (Iuz, Marklands, Scarlet Brotherhood) and the Living Greyhawk stuff and concentrate on the earlier modules. But then again I've been told that I'm an atavistic jerk, so take it as you will. :-)
Depending on how creative you want to get, you could cobble together a very workable City of Greyhawk with bits and pieces from the TSR boxed set, material from Gary's books (especially City of Hawks and Night Arrant), and the new city of Yggsburgh material from TLG. The boxed set maps are atrocious, but feel free to use the hand-drawn maps I've got up on my blog.
As a question of campaign philosophy, you will need to decide on the timeframe of your campaign. This will be important for the simple reason that it will determine whether or not your PCs are competing with the likes of Robilar and Erac's Cousin, or if they are hitting the ground with those legendary personages already having come before. You could, of course, start off in CY 560 and just not have Melf et al involved at all, but that seems to lose some of the fun, to my mind. My own players really loved their encounters with Melf and crew...
You should also bear in mind that "original Gary's vision" is a somewhat nebulous concept, as his campaign changed in significant ways over the years (not that I ever had the pleasure of playing in it, alas...). It started out as a fantastic version of North America, with Greyhawk equalling Chicago. That turned into something more-or-less approximating the published Flanaess. Castle Greyhawk itself morphed from a dozen or so levels to 30 to 6 and now to whatever the plan for Castle Zagyg ends up being. Suffice to say it seems to me that there's a lot of wiggle-room for you as DM, even if you want to remain true to the spirit of Gary's Greyhawk.