Checking the Dell web site, they don't appear to have a driver for any version of OS X after 10.5. Here is a link to that driver.
Note that I was able to install this driver on OS X 10.8, once I got past Gatekeeper, so you should be able to install it on 10.7. Then it should be just a matter of adding the networked printer by selecting IP > HP Jetdirect-Socket for the protocol and selecting the Dell 1320c in the Print Using menu. As to whether it then prints, I will have to leave that to you to test.
any tips for gettings past Gatekeeper
You can Control-click (or right-click if you have it enabled) and then select Open from the menu that appears.
Here's an Apple KB article about Gatekeeper
So everything worked great, got the driver downloaded easily, chose the options that you suggested until I clicked "add" and then got the error message "Unable to verify the printer on your network."
I'm pretty sure I typed in the correct IP address....any other helpful suggestions?
Does it have anything to do with the other options, i.e., the Name and Location box? (I was able to
choose Dell Color Laser 1320c v1.1 in the "Pring Using" box, so at least got that far!)