1998 Nissan Altima Trouble Codes

I am getting the following codes when the service engine soon light comes on in my 1998 Nissan Altima GXE 2.4L engine: P0505, P1490, P1605, P0446. Had Advance Auto run a quick scan and got these codes. They told me the codes mean Idle Control System (P0505), Auxilliary Emission Control (P1490), Auxilliary Inputs Auxilliary Outputs (P1605) and Control System Vent Circuit Half (P0446). The P0505 was the frist code scanned and the others are in order as reported by the scanner. The guy at Advance Auto told me that it could be something in the Idle Control System that is the main issue and the three other codes may be just by-products of the main issue. I am not sure what to check for first and what part/parts need replacing. Please help!