Allocating video ram is automatic.
I have a a10-7850k and the single best improvement for my system was using 2400mhz system memory, the next was adding in another r7-250 gpu..
I used a gigabyte sniper motherboard that could use the 2400mhz memory, not sure about your motherboard.
I use my a10-7850k system for a HTPC and it works wonderfully in that application..
maybe from the motherboards bios