Yep, I'm in about the same boat as you. Got a Droid Charge (1GHz single-core, 512Mb RAM) and am eligible for upgrade tomorrow.
Just gotta wait 'til Verizon gets the damn thing.
Got the EVO 3D on Release day back in Summer 2011 from Sprint. And conveniently enough I'm eligable for my upgrade this Month on Sprint. Anyone else about to take the leap into a new 2 year contract with the S4?