The iPhone 1st generation (iPhone 2G) is more of a straight and flat brick.
and the iPhone 2nd generation (iPhone 3G) has an arch on the back.
Side by side comparision:
the iPhone 3G has more of an arch shaped back:
and i can probably guess that you know that the iPhone 2G has a flat shaped back
In a loose case, such as this one:
the iPhone (2G and 3G) will probably fit in something as loose as that, because they are relatively the same volume; just different shape.
So it depends on if the case that you're looking at is loose, tight, or somewhat tight.
and as for the front of the phone, I doubt it'll work perfectly. You could buy it and try it, and if it doesn't work just return it.