Memory lowgap can be adjusted when you are running a large amount of system RAM and/or a large amount of video ram (typically over 2gb) if you are having issues with all the memory being detected and allocated correctly. 

However, while this can resolve ram allocation issues with GPU's and System ram, it can definitly cause very adverse results with Raid controllers.  If you are running a 3rd party RAID controller that has its own dedicated CPU and RAM all onboard, doing significant adjustments to the Memory Low Gap can cause the memory of the RAID controller to be allocated incorrectly, partially or not at all.  This can cause, almost instantly, an array failure.  The likelihood of this will increase if the array is also the boot drive.

As a general guideline (unless it is the boot drive), be sure all the memory is correctly detected and present, install windows, then build the array after the fact, and enable it under Drive Manager.