More then likely intake manifold. A bad head gasket is very unlikely to cause a leak there. Like cchase, mentioned, it's an afternoon job.
I'd double check my oil, to see if there is any antifreeze in it. I'm assuming your going to change the oil after the intake manifold gasket, so just keep an eye on it for the next few days.
Some intake manifold kits include a gasket for the lifter valley and some just include a tube of sealant. Just use the sealant, the gasket tend to slide out or move. If your set on using the lifter valley gaskets, may want to either rough up the surfaces a bit with some course steel wool or use a punch to put some "dimples" on the block and intake surfaces.