Help with error codes, 01 Trans-Am

This is for a friend of mine’s car:
2001 Pontiac Trans-Am WS6 with 34,000 miles. A4 with headers, a few light bolt ons and nitrous installed by previous owner.
Here are the error codes that I’m getting:

P0140
02 Sensor CKT No Activity (Bank 1 Sensor 2)
P0304
Cylinder 4 Misfire Detected
P0332
Knock Sensor 2 Circuit Low Input (Bank 2)
P0418
Secondary Air Injection System Relay A Malfunction

Okay, so here is what I’m thinking in order to fix the problems, and I wanted to run it by everyone before I went and bought the parts.

P0140, buy 02 Simms. (Where do I get them from, and which one on the car is bad? Aren’t there four 02 sensor’s located on the car? I’m guessing go ahead and purchase two 02 simms)
P0304, upgrade to NGK TR55 spark plugs. It’s about time anyway.
P0332, buy a knock sensor, where is this located and which one is bad?
P0418 What is this?

Any help would be greatly appreciated. Thanks.

02 sims for the rear 02s, bank 1 sensor 2 is the drivers side post cat. Don't know why it's not throwing a code for the other side too - are there cats on the car? Knock sensors are located under the intake, P0332 is for the rear sensor - if you're going to remove the intake I'd replace both. P0418 is an air code - maybe the secondary air was deleted when the headers were installed? If the car doesn't have secondary AIR to get the SES off for it it'll have to be tuned out, rear 02's can be tuned out too at the same time and won't need to buy SIMS. If you want to do some troubleshooting first visit gearchatter.com and look up the codes/diagnostics for his year. Hope this helps....

Thank you for your help. I think that I might have fixed all but the Secondary Air codes. I'll update tomorrow when I scan the car again and than explain what happened