Prebuilt computers are brand name (Dell, HP, Acer, etc)... they usually have the BIOS locked, settings are either hidden or gayed out and inaccessible... So there's not much you can configure.
Overclocking the CPU can also overclock the RAM... see if overclocking the CPU is possible and if so, overclock it just enough so the RAM goes from 1333Mhz to 1600Mhz or around that.