Regarding memory_limit in PHP: Yeah, the required limit is around 12-16 MB. I thought we added this to the install doc a while ago - maybe not.. hmm, I'll take a look and add it if its not there. Anyway, 12 MB (or in some special cases 16 MB) is required, yes.
You shouldn't need 24 MB. Sometimes eZ publish needs a bit more than 12 MB the first time it is run. But this is a one-time-only phenomenon. I believe you need max 16 MB when running "normally" - try to decrease the memory_limit again.
Regarding the database: If the database is empty and the setup still insists on erasing it then simply do it. Could be a bug in the setup wizard. We'll have a look at it. Thanks for pointing this out.