The 737-500 is a "classic" series 737.
The Next Gen 737's start with the 600 series and go all the way up to the 900.
The real big difference between the "classic series 737" 100-500 series and the NG series 600-900 is that the NG series have updated avionics, larger, rounder more powerful CFM engines.
A newer, longer more aerodynamic wing allowing the plane to be far more fuel efficient and fly longer,higher and faster then the competing Airbus A320 series of aircraft (thought I'd plug that
).
Not to mention the updated cockpit complete with either 6-8 large LCD EFIS panels. And an updated passenger cabin reminesant of the 777 with larger over head bins and the such.
All together the new 737's are great but I think that with the new NG's they (Boeing) have done all they can to eek more life outa that 70's airframe.
So they should serious start considering a replacement to the a 737 in the next 10 yrs. or so.
Matthew