I tried the same using snowbreeze cause I was sick of the slowness on the 3G too. Unfortunately now I can't get past the 1604 error even though I did use iREB.

Kinda sucks cause I'm expecting an important call tomorrow morning :S

Anyone know what the issue could be? Tried upgrading from jailbroken unlocked 4.0.1 to 4.1 using custom IPSW from snowbreeze but now all I get is 1604 error on iTunes when I restore (got a ?16? error after iREB but then back to 1604).