Option Overload!

Hi, I'm in option overload.

We started with YM.org and are making a new site YMI.org. Both sites are using the same drupal install base (5.x) with Ubercart.

We want:
- users from YM.org to log into YMI.org without creating new accounts.
- passwords changes to happen to both sites
- I'd like to re-assign roles on YMI.org
- Single signon if possible
What's the best way to do this?

We're using shared hosting at simplehelix.com.

External account management tools: LDAP? CAS? OpenID?
Drupal features: Multi-site? Shared Database?
Modules: Domain Access Module? Singlesignon? FierceSSO?

Just something simple before I pop! Any suggestions, warnings, or advice welcome.

Thanks in advance,

Mike