There appears to only be one model, as far as audio. The front jack is a combination headset and mic jack.  The rear jack can be configured as a Line out or Line in with the included software.  The line in cannot be used for a "computer" mic as a computer mic requires 5VDC and the line in does not include that. Here is the Dell setup document. Precision 3640 Tower Setup and Specifications (dell.com)
I plan to buy a Precision 3640 Tower with audio in/out I noticed on the order page there is basically no information about the audio in/out lacks on the front and back. The picture shows an audio jack in the front, I assume it's a mic in. The rear shows another audio jack, I assume it's audio out. I cannot find anything as far as ordering, to distinguish between a Precision 3640 Tower with audio in/out and a Precision 3640 Tower without audio in/out. How can I know if I'm ordering a Precision 3640 Tower with audio in/out or a Precision 3640 Tower without audio in/out?      
Precision 3640 with audio in/out