Bob,
We pick our one up tomorrow and have been researching this subject until the cows come home.
The 640 x 580 or whatever they have is the pixel size of the image. Multiply those numbers and you get the image size.
There are many cameras in the XT range and it really depends what your customer is looking for. If he wants to be able to view the temperature on each and every pixel in the image then the camera must be radiometric. The more pixels the more ground you cover in each photo. BUT!!! It also depends on the lens size. Bigger lens less area in the photo but much more densely populated temperature information. If your customer is just looking for hotspots then a center metering thermal image may be all he needs. That is there is a temperature scale up the side of the image and the image colors correspond to that bar.
You can spend $2000 and be successful or you can spend $10,000. Its horses for courses. Just some horses cost a friggen lot more than others.