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In an educational situation I would certainly go for a Hackintosh!
I would also install as many different OS´s on it as possible. OSX, Win, Linux, FreeBSD, Solaris, BeOS etc. The more, the merrier!
A competent CS program would teach there is something called virtualization and virtual machines.
Get a copy of VirtualBox/Parrells/VMware and can construct and experiment on numerous 'machines' all from a Mac mini or the MBA.
Tweaking virtual machines has many upsides to them. Can make a copy and if it goes back just throw it away.. ( instead of having to do a complicated wipe and restore-to-previous backup). You can run multiple (smaller) images at the same time it working on a networking program between two computers.
There are some downsides ( video game play). However, more time in video games at high frame rates isn't going to teach you much computer science.
The following is the priority order I would put on a setup given where you started at.
1. MBA (4GB RAM model )
2. A larger monitor with multiple inputs (e.g., 2 DisplayPort )
3. Mac mini with at least 8GB ( 3rd party can do afforable. 16GB will put you back another $300 but wait a year or so.)
(if want to run two Macs and have fallback if MBA has problems ... and vice versa. ). [ can use same monitor for MBA/Mini].
4. A high capacity FW drive array to archive stuff to. (like a file cabinet. )
For example:
http://eshop.macsales.com/shop/Mercury-EliteAL-Pro-RAID
Probably can buy all of that for less than a Mac Pro.
If simply just have are large budget of 'free money' to spend then add
5. A "lab" box. Here you can get some generic PC box that you can mutate for experimental purposes.
[The upside for a "lab" box is that you can get back to more serious work... e.g. Homework , assignments, etc. anytime because that is primarily done on other computers. You can drop the experiment in place if need be. ]
If buying the lab box means working longer hours for "not so free" money or going deeper into debt; then no. Time is a limited asset. There are either supplementary things about your classes , breadth in your studies (other topics), and just regenerative "down time" (socializing with classmates) that much more important than a bigger bling, bling box.