Kevin,

4096 MB should be sufficient unless you want to launch Cloudera Manager.
All of the CDH services should be able to start up and simple examples
should work pretty reliably.

As for your issue, I would suggest going to File -> Import Appliance... and
opening the .ovf file in the directory you unzipped. That file will set up
the virtual hardware for you (instead of you setting up everything but the
hard disk) so it eliminates a lot of variables. Not sure what the root
cause of the issue is, but that's a good first step.