Sigil

Sigil is technically NOT located in the astral plane, nor the elemental chaos. There are various known and accessible ports however, many in the astral realm, with secured two-way portals that allow access into the city.
http://www.wizards.com/dnd/images/Map_DMG2/5.jpg

Thats a map of sigil. Keep in mind the city is a giant ring that looks like this:

http://www.shadowland.org/Planescape/images/sigilring.jpg

With the buildings and streets and whatnot on the INSIDE. There is also more on Sigil in the DMG2, Manual of the Planes, and various dragon magazines. It is likely that we will be using Sigil as a hopping off point for a lot of the adventures.

Links
Spelljammer
Astral Sea
Deities and Domains
Elemental Chaos
dimensions and different realities

Sigil

Spelljammer 4th Edition LukasLevy