Check engine light

Hi all,
I have a 97 ES300 with 82000 miles in it. Few weeks ago, I opened up the air filter housing to check the condition of air filter. The next morning when I was going to work I saw check engine light came on. The next day, when I was taking it to the dealer for check up, the light went away by itself. So I brought back from the dealer and thought that air filter housing might have caused that light to come on. But the next day when I was returning home from work there it goes again. So I took it to the dealer for check up. They said it was o2 sensor on bank one that is bad. However, there was warning about o2 sensor on bank 2 as well. Teh guy at the Dealer said I should change the o2 in bank 1 first to see if the problem goes away. I tried that but it came again after couple of days. After reading this forum, I took the advice to go to Autozone for scanning. They scanned the car for error code and found two errors indicating that o2 sensor is bad. It does not say specifically bank 1 or bank 2. They reseted the light so that I don't see that check engine light again fo a while. Now it has been three days and I did not see the check engine light again. Now my question to you experts:

1. Which one is bank 1 and bank 2 sensor? Where are they located?
2. I already paid $376 to replace the one in bank 1. I hate to pay another $276 to replace the 2nd one. Is it easy to replace the 2nd one if it is bad. How can I tell they are bad other than computer code? Are they easily replaceable? I am a handyman. Ususally, I can fix stuff with limited direction.
3. Can you replace them with after market bosch o2 sensor?
4. If I keep driving it until the check engine light comes back again, is it safe to drive (except bad gas mileage)?
Thanks.
Mo

Bank one is the bank next to firewall. the A/F ratio sensor is located at end of exhaust manifolds..It will take a 7/8 inch open end wrench to remove old one.the dealer told you that both sensors are bad because they really don't know which one is bad..P0171 bank one lean is the A/F sensor on the firewall side..You said somthing in your post though that you will have to double check..Take a look at your airfilter box there is a couple of rubber lines that connect to it make sure you did not disconnect these lines while you were checking your air filter..also one of the lines is very small on the upper portion of the box towards the front of the box make sure you did not break it..when you go to the parts store check advanced auto parts for the correct A/F sensor..have them match the number on your old one..auto zone has trouble finding the right one.

Hope this helps..
Matthew..