Another resource is the Airliner Modelling Digest at Yahoo Groups. Under the files section (you'll have to register to have access), there is a veritible cornucopia of conversion information.
http://groups.yahoo.com/search?query=airliner+modeling
The basics of the Boeing 737, and the kits to use in 1/144 scale:
The -200 is a lengthened -100. Use the Airfix kit with fuselage mods as required.
The -400 is a lengthened -300 and the -500 is a shortened -300 (the -500 was intended as a -200 replacement, or so I once read). Use the Daco kits, or the Minicraft kits. Daco's fuselage is shaped better....
The -600, -700, and -900 are all based on the -800 with shorter or longer fuselages. Use the Revell AG kit, modified as appropriate.
In order to build a -700 in 1/144 scale, start with the Revell 737-800 kit and make the following cuts to the fuselage:
Forward of the wing, but aft of the baggage hold door: Remove 18.7mm (0.74")
Aft of the wing, but forward of the baggage hold door: Remove 19.75mm (0.78")
Remove the tail bumper and fill the hole. Add filler as needed.
Now, if you want to make life easy, check out Contrails Models:
http://www.drawdecal.com/contrails.htm
The choice is yours....
In 1/72 scale, Welsh/Proteus did some mixed media kits of the MidGen 737's that were quite nice.
Aurora did a so-so 1/72 kit of the Classic 737.
I don't think anyone has done 1/72 kits of the NextGen 737's yet....
Hasegawa has done several 1/200 737's, ranging from the -200 to the -300 or -400.
I don't do narrowbodies in 1/200, so someone else will have to elaborate.
Ralph