Here is a Do-It-Yourself (DIY) thread on changing the air/fuel sensor. You usually have to change it if the check engine light (CEL) is on and the car is throwing codes P0171, P1130, P1150, etc.
I don't want to get too much into whether you need to replace it or not. There are other threads that do that.
My car's CEL went on. I stopped by local AutoZone or Advance Auto and had them read the code. It was P1130 - bank 1 sensor 1. That's the a/f sensor on the back three cylinders exhaust manifold (rear). This DIY will cover replacing the back sensor.
If you need to replace the front sensor, it's similar just in a different spot. :-)
The Lexus part number for the RX300 air/fuel sensor is 89467-48011. The OEM a/f sensor is made by Denso and the Denso part number is 234-9009. The two are equivalent.
Lexus sells the part for ~$300. Toyota sells the part for ~$200. You can generally find one on eBay for ~$150. I found the part on Amazon for $126 with free shipping; by far the best deal. I ordered it on a Monday and received it Wednesday.
Here is the Amazon part for reference (Sept 2009). If this link dies in the future, just search Amazon with "Denso 234-9009" and you'll find it assuming they still stock it.
The previous photo shows the a/f sensor wire plugging into the socket. I found it easier to get to the plug from the top through the engine compartment. Just reach back and press the little tab down and pull the plug out. Hopefully it won't be stuck on too hard.
There is a couple of tools one can use to remove the sensor. I used a 7/8" wrench with a nice elbow in it as shown in the photo at the far right. I found the elbow to be very helpful.
To the left of that in the photo, is a sensor socket with a breaker bar. The socket is open on the side so it can get around the wires. The socket is 7/8" as well.
The last tool in the photo on the far left is a straight 22mm wrench. Although the tool fits, I was unable to get it to unscrew the sensor. There's a metal bar in the way which is avoided by using the wrench with the elbow in it.