Secure enough for what? You should make sure that you have a high enough entropy when generating the keys. This can be problematic without disks or internet connection.
Secure enough to not have to worry of having thousands of dollars worth of bitcoins (or whatever the value of bitcoins may be in the future) being compromised by bitcoin searching hackers.
As far as entropy, I would use the page bitaddress.org (save local copy), is the entropy good enough from that? (it creates it by random mouse motion and also user input characters).
Sounds like a reasonable secure plan. The software you picked is old and tested, has a entropy source that is independent from the OS and its designed to run offline. Make sure you verify the download though.
Great, thanks. I might give it a try. How would you suggest verifying the bitaddress.org download?
via PGP -> https://www.bitaddress.org/pointbiz_bitaddress.org.asc
-> https://www.bitaddress.org/bitaddress.org-v3.2.0-SHA256-ad4fd171c647772aa76d0ce828731b01ca586596275d43a94008766b758e8736.html.sig