oxygen sensor replacement

I've been noticing my 1996 Blazer has been lacking power lately so I decided to check out the O2 sensors, the readings from Bank 1 sensors 2 and 3 are erratic from .2 to .8 volts while sensor 3 is steady at about .65 and then theres bank 2 sensor 1 which is also erratic. Looking under the chasis I think I can identify 4 oxygen sensors, one on each exhaust manifold before the Y, and then one after the Y just before the catalytic converter, then a 4th after the catalytic converter, is this right? I'm assuming that bank 1 is all of the sensors on the right side of the vehicle and the one sensor in bank 2 is the sensor on the left manifold.
When looking at discountoxygensensors.com I see they sell only pre-catalytic converter and post-catalytic converter sensors, so are all 3 of the ones before the converter the same part? And do I need to replace all 3 of them if they're giving bad signals?

Thanks in advance,
-Drew

OK, on the 4.3L and 5.0L engines with single cats, the two sensors closest to the engine are the pre-cat sensor type and the two closest to the catalytic are post cat type. The main difference is the plug style, I believe the post cat sensors have a square plug and the pre cat sensors have a rectangular plug. If you're looking for a little performance boost you just need to replace the two sensors closest to the motor

Bank 1 Sensor 1 and Bank 2 Sensor 1 should change rapidly going above and below 450 mv. Bank 1 Sensor 2 should change some. Bank 1 Sensor 3 should have less activity than Bank 1 Sensor 1 or Bank 2 Sensor 1

so your saying the one just before and after the catalytic converter are both post-cat sensors?

yes they are, you can verify by checking the connector on them