Hate to disagree with Aztruck but...
If you do a conversion course on a 737-300 your licence will be stamped 737-300/900.
However, an operator will require you to have experience on the relevent variant. If you have completed a -300/400/500 conversion, you will require an OPERATOR's difference course to operate the NG series (700/800/900)
The 737-200 is now deemed to be a different type and requires a conversion course , even if you have the 737-300/900 on your licence.
H