I have owned both the 640 and 640-1, and prefer the 640. A friend prefers the 640-1 (he bought mine). Both are excellent revolvers, you can't go wrong with either one. I prefer the 640 for it's slightly smaller size and weight. The 640 is fine with .38 +P, I carry 135gr GDHP in mine.
The 640 was introduced at the beginning of 1990. The first of them had a CEN serial number prefix. Many of the CEN guns (including mine) are also marked "Tested for +P+". Around '95/'96 Smith brought out the Magnum J frames, including the 640-1. There was also a 640-2 which was .38 Spl only on the magnum frame. I believe that was made for the NYPD. The 640-3 marked the arrival of the internal lock.
Both the 640 and 640-1 are hard to find. In the last 3 years I've seen two 640's, and I bought both. One I got in trade for a 36-7, the other for $475. The 640-1 I had also cost me $475. I'd say that if you run across either version grab it, they aren't making them anymore.