Welcome to historical wargaming, theHandofGork.
I'm personally not into Ancients/ECW (yet), so you may want to take my answers with a grain of salt.
1. What rulesets are most commonly used?
As far as I know, the most common and "traditional" rules for Ancients is DBA (
De Bellis Antiquitatis).
Wikipedia Link:
http://en.wikipedia.org/wiki/De_Bellis_Antiquitatis
However, WAB (Warhammer Ancient Battles, published by
GW) seems to be gaining quite a lot of followers recently. Since you're coming from a
GW background, this may be a more comfortable option for you.
2. Which are the best, and why?
I don't think I'm qualified enough to answer this question. I'll skip this one.
3. Are there forums for historical gamers?
You bet! TMP (The Miniatures Page) has an excellent forum, which
IMHO is the best there is for all historical wargaming.
Link:
http://theminiaturespage.com/boards/
Any players (or stores) in the South San Francisco Bay Area?
I don't live in the States, so I'll have to skip this one too.
Automatically Appended Next Post: Oh, I forgot to add FOG (Field of Glory). This seems to be an up-and-coming ruleset that covers Ancients and Medieval warfare, published by Osprey.
Official Link:
http://www.fieldofglory.com
From what I hear, quite a number of gaming clubs are switching from DBA to FOG.
Automatically Appended Next Post: With regard to question #2 (Which are the best, and why?), I've managed to find pretty comprehensive reviews for both DBA and FOG.
Review of DBA:
http://www.deepfriedhappymice.com/html/rd_dba.html
Review of FOG:
http://www.deepfriedhappymice.com/html/rd_field_of_glory.html
As for WAB, I think you'll already be aware of its strengths and weaknesses, as it is essentially a tweaked version of
WHFB 5th Edition, with all magic-related stuff removed.
In any case, I believe the TMP boards will be your friend if you're going into historical wargaming.
Cheers!