I just graduated in the spring of 2007 as a mechanical engineer and I got one of the sun-flower iMac G4's as a high school graduation present back on 2003.I had that until the summer between junior and senior year when I bought myself a new Intel iMac. From my experience, I would get the computer that you'd most want for daily use for the next 4-5 years of your life. Generally, most good engineering schools will have one if not several dedicated computer labs full of computers no older than 4 years running all of the specialized engineering software you'll ever need. You may find that as you get further into your 4 years there, you'll be spending more and more of your time in the engineering building, because you'll be doing work in a lab you need or because you can work with your classmates and because it is a lot more conducive to getting stuff done than your dorm room. Most of the software that the school would be running, is expensive to purchase. Some often have special student deals, and aren't too expensive, but most still are.
Basically, other than writing up mind numbingly long and large amounts of reports for your classes, there probably won't be much other work you would want to do outside of the computer labs. In 4 years, I only twice REALLY needed engineering software on my computer... once was freshman year I went out and bought the student version of MatLAB because I needed more time to write a program, and then when I bought the Intel iMac I ran SolidWorks 2006 on Windows Boot Camp so I could do work at home for my internship.
So basically, get a Mac, you'll love it. For the engineering software, 9 times out of 10, you'll just use the computer labs!
Good luck with engineering, and enjoy your new Mac!
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