Hi tempusfugitive,
Unless you are a file system guru or have system software experience, i.e. you are a regular user, stick with ext3. The difference between ext2 and ext3 is that ext3 has a journal capability.
Suggest you read:
ext3, and the See Also sections before the end.
The author of ext3, Dr. Stephen Tweedie, I believe works at Red Hat, and there is an ext3-users mailing list. The replacement for ext3, namely ext4, is in development.
Your use of a ? at the end of the next to last paragraph of your message does not seem to indicate that you are sure whether you will be able to boot from an EHD - just an observation - are you asking that question?
-- Tom