Bluemoon,
We were all once in your shoes (and it was mightly crowded).
I would suggest any of the following fine campaign settings:
1.
Arcanis/Onara from Paradigm Concepts: with the added bonus of a "Living" campaign pumping out modules and support.
2.
Portals of Midwrathe by Twilight Ink: a small, shared campaign world which can be found at
d20rpg.org (my personal favorite).
3.
Erde by Troll Lord Games: a bit cheesy but with a lot of heart.
4.
Kingdoms of Kalamar by Kenzer Co.: by far the best production values of any campaign being produced.
5.
Hârn by Columbia Games: quaint and well produced with all the flair and grit of the real Middle Ages.
6.
Middle Earth by ICE: lots of cool maps and a campaign setting that is pretty easy to understand (I don't recommend running it withe the MERP system though...too cumbersome).
7. Old
Glorantha by Chaosium: not the new stuff by Issairies Press--it is a little too weird.
I would hesitate to recommend something like
Greyhawk unless you are looking for nostalgia. And as an old crotchety gamer I haven't been able to keep up with all the new stuff so I can't really speak to those.