This job is partially done but thought I'd share.
Rented a floor grinder from Lowes/Sunbelt tool rental. Overall the machine was easy to use. The only 'gotchas' were having to just go slow and spending a few extra bucks on the Dewalt diamond angle grinder attachment clean up the edges.
The garage floor is now smooth like a pool deck; when the epoxy is laid down it should have -no- problem soaking in and curing to the floor.
Sam doing work, front loader left -heavy- tire marks
http://farm8.staticflickr.com/7069/6966894199_de5b45d77a_z.jpg
Marks at the top, also there is a light coating of sealer
http://farm8.staticflickr.com/7045/6821663816_50726c9bdd_c.jpg
That black area after grinding
The angle grinder with attachment