Good Question.
There are now 3000 domain name extensions available. Pay more and stick with the big guns
.COM, .CO.UK, .COM.AU you can make money on others but it is risky.
Check this out
www.dnjournal.com and it gives you an idea whats selling in what extensions.Some extensions are $1 USD a year now.. They are giving them away basically.
Some people here on the forum have some good 2 and 3 letter .com.au results and are keen on them.
If you could get any 2 letter .com's you are probably sitting on a goldmine. That ship may have sailed.