Those portable foam boxes are ok for voiceover work on a budget, but quite frankly you can line the interior of a quiet closet in your house and use that just as well (walk-in closets full of clothes and carpet on the floor are awesome voiceover booths on the cheap). Where you really want to spend money wisely is getting the best microphone you can afford. I suggest going to a local music store that has a decent recording audio selection, and audition a few mics they have to find one that really suits your voice. Get a pop filter while you're at it.
Although all-in-one USB mics are available, their selection is rather limited to the cheaper voiceover-type mics. You'll find better selection of mics that plug into a separate mic preamp that in turn gets plugged into your computer's USB port. Get a decent mic stand and shock mount. Finally get a decent set of headphones that plugs into the same mic preamp/interface box (or the headphone out of your computer) so that you can play back your recordings, and/or listen while you record.
You don't necessarily need to use garage band, though many do. Give Audacity a try, see if that keeps the fan from spinning up. Otherwise you can remotely locate the mic, preamp, and keyboard/mouse away from the computer so fan noise is not an issue.
Here's some suggestions to get you started, though these are from Sweetwater for convenience sake, go support your local music store with your pocketbook, I'm sure they'd appreciate it!
Lots of mics to choose from (this is where you want to spend a good portion of your money, but audition them first before spending $$$):
Preamp/computer audio interface options if you don't go the USB mic route:
Stands (depending on whether you do this at a desk or freestanding):
Other accessories you might find useful:
Hope this helps!