what is that little plastic tank that is bolted underneathe the intake manifold? also when installing a cai where does that house go that was at the bottom of the stock intake?

 

Take a pick of both but you might be talking about the charcoal canister.

 

It is the Intake Air Bypass system...
Lemme guess you have an H22 or H23?

IAB butterflies is what its commonly called.
The tank is a vacuum tank that works to apply vacuum to the IAB vacuum port, which rotates the butterflies in the 2 stage intake manifold.
Low rpm uses the long rounded intake runners for more torque and better airflow at low engine speeds. At a certain rpm, (6000 in a GSR, prob like 4800 in your case) the ECU sends a signal to the solenoid, which blocks the vacuum from the IAB vacuum port, enabling the shorter more direct runners to supply extra airflow at high engine speeds.

If you were to remove the vacuum line to the IAB vacuum port, you would notice much less torque and responsiveness at low rpms.

It gets vacuum from a port on the back of the IM, so installing a CAI will have no effect on the system.