Any suggestions on a rear sight for a flat top upper? i'm going to eventually drop some optics on it, but until then i would like to be able to shoot it.

You need to decide if you want a flip-up BUIS, fixed BUIS, or just get a carry handle.  

Some of that depends on what kind of optics you plan on using.  Pretty much all your rear sight options float around the $100 mark.

I really like the ARMS#40.  If you will be getting an acog, get a #40L.  If you want a fixed sight, look at the LMT or the Larue.  There are tons to choose from.

Or, just find a used, cheap carry handle, if you arent going to use a BUIS... like if you will be using a standard scope.

AMEN!

you can go cheap carry handle now and later cut he handle part off and make a fixed BUIS.

If you know you are going to invest in optics (a good decision, I might add), you need to figure out what optic you are going to use.  This will drive your decsion on a BUIS.

I prefer the Aimpoint with an LMT fixed BUIS.  For an ACOG I'd go ARMS 40L or Troy or KAC 300m.  For a precision rig I'd go with the KAC 600m.