Okay, so I don't know if this is even where I should be posting - but I got a really creepy email yesterday from some email address I don't know. I have an idea of who it might be, but I'm not certain. I also have an email from this person at the address they usually use. Can I compare IP addresses? .. Or am I just way out of it on this one?

You'll have to find the ip numbers on your own, but samspade.org is a good tool to use once you get an ip.

Try http://www.dnsstuff.com/

An IP address has almost nothing to do with an email address.
I can get or use various IP addresses and still use the same email address.

For the record; this post points to a WHOIS client that will document WHO registered for a
specific IP address or address range.

Back tracking an Email message is messy and subject to going no where, due
to email spam. Most email client software does not show all the headers used
to deliver a specific message to you. The basics are:
take the From: xxx@domain.com and use WHOIS or NSLOOKUP to track down the domain.com part.