I am currently running out of ram on my 2GB laptop. I generally max out at about 3.5GB of address space in use. That is what I get for running Oracle, WebLogic, Eclipse, VisualStudio, our custom app that needs 1GB itself, Mail, a news reader, and a few other supporting java and .Net apps all at once.
Once my MBP arrives I will use MySQL rather than Oracle for most of the development work, with Parallels for final Oracle testing, and for VisualStudio and .Net apps.
You'll probably find PostgreSQL a better Oracle replacement, though on OS X it's more involved to get it up and running. I'm not "advocating" particularly - it's just that PostgreSQL datatypes, procedural language etc etc are a better match to Oracle. If you're doing ORM with say Hibernate or JPA then I guess it doesn't matter so much, & MySQL comes with some nice dev tools