P0153 - Which is Bank 2?

1998 Windstar - 87K miles.
I currently have the engine light on with Code P0153 - Upstream heated O2 sensor circuit slow response (Bank 2). Which bank is bank 2 anyone? - Is it simply the one containing cylinders 4,5 and 6 i.e towards the front of the vehicle?
I've reset the light a couple of times but it's came back on. I bought a new Bosch OE sensor yesterday ($50 at PEP Boys) and am going to replace the supposedly defective one???
(The History - Seems unrelated but last weekend we were on a trip about 250 miles away from home when I had no choice but to drive over something on the highway. A loud thump and I pulled off, stopped and on inspection found the transmission pan damaged and fluid leaking. I ended up towing the van home on a uhaul tow dolly with the front wheels off the ground. During the week I fitted a new pan and filled with fresh fluid and the transmission seems fine but out on a test drive and I get the engine light and P0153.)

You have the right bank. Bank one is always on the same side as #1 cylinder.

Upstream is before the catalytic converter.
A "oxygen sensor socket" will help....It is a deep socket with a slot on the side to accomodate the wires.
Spray a little PB Blaster on the threads....and let it soak.
When you install the new one.....put just a little antiseize lubricant onto the threads.
Make sure that the wires are routed such that they do not hang down....to be caught, or that they do not come in contact with any exhaust component.

I would almost want to replace both upstream sensors.....so that....in theory....they will have the same response charactoristic.

Thanks for the quick response folks and of course the advice and information.
I just bought the one sensor for now until I satisfy myself that a 'duff' sensor is the problem here then I'll take the advice and replace them in pairs. Surely you have to agree that it's uncanny getting the code after the incident with the transmission. I'm thinking the sensor maybe somehow got damaged in the inpact, douched with transmission fluid or the cable strained?
I did buy the proper socket as I have read a fair proportion of the threads on this site at one time or another and indeed the former Cartrackers.com.
Thanks again for the help it is greatly appreciated.

Well I replaced both upstream O2 sensors and the check engine light hasn't came back on after quite a few trips so all seems well. Thanks again for the advice this really is a super resource for Windstar owners.