There are a couple of spreadsheets floating around in the single table tournaments forum that have calculators to determine confidence intervals on your results.
AleoMagus spreadsheet (from One Table Tournaments forum)
http://www.tilthappens.com/files/2Plus2SNG.zip
I plugged your #s into the first spreadsheet and it spit out a 99.6% confidence level that your avg $ profit/tournament is positive. I'm no stats guy, so you may need a greater sample size to make these calcs statistically valid. But it sure looks like you can handle the $20s just fine, though- congrats.
If you do get the itch to go up, my advice would be to go slow and maybe set aside a small fixed chunk of change for the $30s. Also, the idea of playing just 1 table at a time to focus & accelerate your learning is a good one.
FWIW, I think there's a much bigger jump from the $30s to the $50s SNGs on Party than from the $20s to the $30s (one cause being the 1000 starting chips vs 800). I'm trying to scoot quickly up the SNG ladder for fun and currently stuck on the $50s:
SNG Ladder Challege update