As far as I'm aware Roku channels can't specify whether they use the internal storage or any inserted MicroSD card.
It's a USB slot and I have a USB thumb drive in it
The MicroSD card slot is immediately below the HDMI port on current generation models. (For the three 2011 "Roku 2" -- HD, XD, XS -- models, it's immediately above the HDMI port.)
All 3 of my Roku3 have sd cards. I have a 2 gig card in each one.
How many channels do you have installed? And how many of those are games? I've got around 110 and the internal storage is sufficient, but the only game I have is the Angry Birds one which came pre-installed.