The hardest transition for me to make was from $100 to $200 NL. Going from $200 to $400 was a smooth transition for me as I didn't find the play to be appreciably different. Now that I'm playing a bit more $600 I haven' t found the play to be drastically better either. Like Gabe said, I've heard there is a pretty noticeable difference between $600 and $1000NL, but I'll try to conquer that hurdle when I come to it.

I also encourage you to move up as well. As Smackin mentioned, taking shots at a higher level makes the transition to that level easier when the time comes. It also can give you more confidence in playing your normal limits. I'd also like to echo Smackin's comment about the money you can make playing even just $400 NL. A solid player can fairly easy make $100K+ in a year at this level.