As someone who has formed a hardcore-ish raiding guild from the ground up (we started as 3 rerolls from Illidan back in December '05), I thoroughly agree that having a good base of talented and dedicated players is the key to a successful transition. We had about 10 exceptional players as our base and we slowly grew from there with ZG being our jump-off point. We simply took friends, random folk, and people looking for guilds on our early ZG runs and tried to impress people with our degree of competence and fun attitude. It worked very well and word spread like wildfire. We were lucky enough to have some very eager and willing people apply to the guild and we were extremely selective at first so we could build on the core group. By the time we reached the end-game raids, we had a very awesome core of 25 or so players and the other players fed off of the core's enthusiasm, knowledge, and attention to detail which in turn produced other very good players.

Also, I want to re-emphasize the need for you to a) establish rules early (yet allow flexibility), b) create a core group of players of what you envision your guild to become, and, most importantly, c) create an atmosphere where people are willing to join your guild and come to raids for reasons other than just loot or progression (though both are definitely factors). Don't be a dick, don't burn bridges, and realize that guild leading is nothing but PR and HR.

Best of luck!