Cutting some holes in the case

You can cut holes in metal with a jigsaw, yes. I've done it a few times, just make sure you're using a metal blade! You get blades meant for different things; don't mix them up, disaster can occur. (Well the worst that happens is you ruin a blade).

You will have to make an initial incision with either a drill or a dremel to begin with, then it should be smooth sailing for the jigsaw.

If you're looking to make a fan hole, I really don't recommend using a jigsaw for that; it's going to be all kinds of difficult. You could use either a dremel with a reinforced cutting disk or a hole saw to cut fan holes in your case.

4.5" hole saw is what you need for 120mm fans, and the screw holes to mount the fans should be 4mm/ 0.15".

You can use wood blades from a jig saw to cut a hole in your case. That's how I cut my window anyways, 'cus the only metal blade he had broke

I would say use a dremel if you're not going to be cutting a circle. But if you're going with a circle, try for a hole saw. If you can't get a hole saw and only a dremel, be sure to hold it at a 45 degree angle so that you can cut smoother and whatnot.
Jigsaw is best for long cuts, dremel is good for starter cuts or for making little holes like for cable management.

Personally, I would get a hole saw that matches the hole size you want cut. It will take less than 30sec to get a hole done with one. For cable management holes a hole saw should run you $20bucks.