Ok.
Your internal domain name can be anything BUT make sure the internal main name is not taken in the Internet,just in case.
Second: I'd recommend you stick with the .com extension for your internal domain. Your web server, your company's internet presence FQDN has nothing to do with your internal domain name unless you choose to do so. This is demonstrated when you install exchange 2000 which adheres to use your internal domain name as default for email address suffix but you can modify the SMTP address within exchange system manager to reflect your company's internet name, or whatever the MX record is.
Also, your web server might already be using an ISP DNS svr. Whihc might mean u are taking hosting any EXTERNAL DNS SERVER for that web server, it's completely outsourced. being that the case, you install your own internal DNS svrs that can forward name resolution to the ISP servers.
Good Luck!