Master-Master replication or Master-Slave? (no replies)

Here's my problem.

I have a website hosted on server A which uses database A for content storage. I want to have a backup server B which will kick in and provide content for visitors if for any reason server A goes down.

So what I need to do is to synchronize the databases on the 2 servers but with a few things in mind:
1. Whenever something is modified in database A (via INSERT UPDATE DELETE etc) push the modifications to the database B on server B
2. If server A goes down and site runs on server B, if there's any new modifications on server B (INSERT UPDATE DELETE) push data back to server A as soon as it comes back online

What I want to do is have the two databases in perfect sync without a lot of delay between syncs.

Any pointers to how I could do this?

Thanks