Limit poker is huge on variance. Many mid-stakes pros recommend 500 big bets for 20-40 & 40-80, 8-16 is probably weak enough to beat with less, but if you are playing for a living, I wouldn't recommend it.
As far as 3-6 and 4-8, you might be able to beat the drop under the right conditions, but it won't be for much.
I played part time for quite a while, mostly 20-40 and have friends who have become quite successful, and feel confident I'm speaking the truth. Anyone that wants to study the game, I'd recommend spending time at
twoplustwo.com and checking out
deucescracked.com for great instructional videos.
disclaimer: I'm not involved with DC, but I do know one of the founders, "DeathDonkey"