How bad is it to run an ancient Mac Pro with an unsupported OSX? I have an ancient (c. 2007?) Mac Pro running OSX 10.7.5 at work. It's so old, Software Update no longer allows me to update to a newer operating system. I use a lot of software provided by MacPorts, and I'm finding I'm having issues that may be related to not being able to update. I'm no longer able to update certain software, either. I use it to ssh to when I'm not in my office, so it's open to the network (though I expect that it is protected to some degree by my institution's network).

Unfortunately, I lack the funds to purchase a newer model! 

I know running an outdated computer has problems: 


Software vulnerabilities don't get patched, exposing me to hacks (eep!)
Possible instabilities with 3rd party software that might install on my system but not work with my system. 
The most current versions of some software packages won't install
I miss out on some of the new features of the newer OSs (e.g. Messages)


I'm primarily concerned with the first problem listed.

Question

What are some other risks, and how can I evaluate and/or navigate them? Is it really, really bad for me to run it like this--so bad that it might be advisable to install and use Linux instead (as described here)?
If you have the 2007 Pro 1.1, 10.7.5 is the end of the line for you. But depending what you need to do, it's not all bad. I have a 2010 pro and run only Snow Leo, 10.6.8 most of the time. 
The only thing you can do is monitor what apps no longer function, and save up for an upgrade. $600 or so to jump to 2010 pro.