(Steps into the other room to look at my book.)

#20 is a done-in-one story that was a little bit of a fill-in between two 3-issue arcs.
So Nirvana will fit either before or after #20 without a problem.

'Reading' / 'Reading soon' stack: DC/Vertigo - ArtOps, Batgirl, Gotham Academy, Gotham by Midnight, Injustice; Marvel - Allred's Silver Surfer; Other - Abe Sapien, Descender, Fight Club 2, Goon, Mind MGMT, Southern Bastards