Quick way to split a map in half?

I downloaded the entire state of Nevada, plus some of CA just to keep things squared off.  Now I have all topos down to 1/4 min.  No ariels though.  The file is 5 GB in size.  My SDMC is only 4 GB and it already has a couple gig of data on it.  So now I want to split the file up into managable regions.  Is there a fast way to do this or do I have to start downloading into new files again?

I have tried to locate a 16 GB Micro SD card in a class 6.  All I can find is a class 2.  So I ordered an 8 GB in class 6.  When that gets here I can put the whole state on one card.  But for now, I'd like to break it up.

Nate