O'K let me explain process, so next time you will be able teach others -):
Don't know why you stop your choice on LM3900, but first things to do it to
find data sheet on chip.
I like: http://www.alldatasheet.com
There is a big chance it will include application drawing, formula for calculation gain
parameters, power supply min and max, noise etc, etc.
(I've attached a pics and docs).
You will save a lot of time, because all this application details from manufacturer
engineer, who design a chip itself, anyone else knows better than they are?
Regarding LMP2021, I just came across it looking throw National Semiconductor
"Industrial Sensor Interface Solutions Guide national.com/industrial" document.
Doc size 1.4Mb, can't post here. Google pops up it for me on
"industrial differential amplifier" search pattern.
If for some reason IC difficult to find on a market, look on others "big monsters" sites,
TI - Texas Instruments, Linear Technology and so on.
Name of the guide and section ask on google, bastard knows everything -):