I'm having a hard time figuring out when people asked what model I have. I have a 325i (2003 model), but what are the numbers like E36 and E30 ?

2003 model will be E46
'91 to '98/'99 (I think) will be E36
pre '91, but not sure of duration, will be E30.

they are the engineering series no. Yours is a e46.

For chassis and engine codes, go to the Bavarian Autosport site: http://www.bavarianautosport.com
Click on the Tech Info link at the bottom of the screen, then scroll down to the Tech Tip section and click on the BMW Chassis Codes link.
-Josh