Yeah, it goes into a spare slot. You should also have some active cooling on it, as it will get rather hot when pumping that juice through. It also comes with a motherboard compatibility chart, which has the motherboards and max voltages with those motherboards that they have tested. I was told over at XS that you have to do a VTT mod to the mobo, but I dont think OCZ or anyone else would sell something that you can only use after you mod your mobo, dosent make any sense.
What motherboard do you have? Is it the DFI Lanparty? They already supply up to 4 volts to the RAM, so the DDR Booster would be redundant and a waste of time and money if you bought it for use with that board.