[Yeknom58]

Bah,

40 is pretty conservative for someone who doesn't play for a living..20-30 should be fine. You just want enough such that if you hit a rough patch where you suddenly drop like 7-10 buyin you can handle looking at your cashier page.
Oh and as a rule of thumb don't move up to the next limit until you feel totally comfortable losing like 2-3 buyins over the course of like 10 hands. You don't want a few bad beats to get you all bent out of shape. When you first move from 50-100 and you lose like 300 you can feel pretty sick.

Also, when I moved up in limits I would play a few thousands hands between limits. Meaning I would 3 table 25NL and 1 table 50NL, then 2 25NL and 2 50NL...then etc..till I was only playing 50NL.