fiber re-enforced cutting wheels. If you try the non-re-enforced they might shatter and mame you lol. I used them to cut a fan hole in my antec sonata which was steel.
That's the same as I use when I have them, the regular ones work ok as well, just need to be careful and were the correct safty gear when working.............
Wal-Marts here carry just about the full line of Dremel accy.......
The tool I've used the most is a 1/4 grinder. Even if I'm going to be breaking out the dremel, I find removing large sections first makes things easier to work with. To put my case window in, I made the long cuts with the 1/4 grinder using a very thin blade. I used the dremel only in the corners.
The 1/4 grinder can do allot more damage to both the user and the computer. So I guess it depends on your skill level.
if I were doing it, I would just use a jigsaw and then a sanding/deburring wheel on the dremel... but of course, I have a jigsaw laying around. Again, if you use the dremel, be careful. Breaking a cut wheel will produce several small pieces flying all around at high speed (speaking from experience here).
Hole saw pure and simple. That is, if you have a drill... You can cut a near perfect hole in about 2 min with one compared to the 30+ min it would take with a dremmel.
If you really got the bucks, a hole punch/slug punch is even better but those will cost you about $300 used on ebay, 20" socket wrench and socket included...