Mikey5oh,
S&W has upgraded the quality of their guns as far as +P ammo is concerned. That does not mean there is anything wrong with your 640, it just may not be as strong as later guns due to the metal used, heat treatment and any changes S&W has made. I would stick with the "standard pressure" ammo. It is what I have done with my alloy model 12 which is a 6 shot K-frame gun with an aluminum frame and on my 5 shot revolvers like the model 36, 38 and CHARTER ARMS Undercover.
As for NYPD, when I joined my agency, not the NYPD, they issued +P+ 9m.m. to anyone carrying an approved, personally purchased pistol, like the GLOCK 17 or 19 or a SIG 226. NONE OF THOSE GUNS WERE RATED FOR +P+ because there is no standard for +P+, but they wanted us to carry ammo that was effective and if it wore out our guns sooner, well that was the price we paid. Later on, when we went to the .40 S&W, the very effective ammo that we used wore out our BERETTA 96's, in 10 years. I carried mine for only 8 years, before having to turn it in.
NYPD was issuing an ammo it considered effective after having a lot of union problems over the 158 grain lead round nose, aka: the widow maker. Your gun will not blow up, but it will shake itself loose to the point it will not function.
I use HORNADY 110 grain FTX load, the standard pressure, not the +P. I have tried the WINCHESTER Train & Defend, but have had to many problems with WINCHESTER ammo not firing in my S&W'S. This is not a problem in my RUGER'S, I do not know why.
I have also tried COLT and DOUBLE TAP 110 grain jhp standard pressure ammo in my model 12. They are supposed to be safe, however, flash and recoil were very noticeably increased.
I am not sure that there is a really good performing standard pressure load for a 2 inch barreled .38 Special.
Good luck with your search,
Jim