My father plays with Arduino as well as Raspberry PI's. I fiddle some with PI's, but have no experience with Arduino.
I am a Linux user though. I am writting this on a Lubuntu system I built and I find it to do what I want it to do, and be light on resources. I used it on a netbook, booting from a USB stick for around 5 years as a primary computer. With the costs of USB sticks, personally I would recommend an 8gb one (dvd sized). You can find out what size your distro will take when you download it and I try to leave some space for the other things you might used with Arduino. Lubuntu is a Debian based system, with LXDE (same desktop as the PI, in case you get into them), but on a x86 based platform. Raspbian, is one of the PI distributions, but based on ARM, architecture (think the OLD RISC Mac's, verses Windows machines, sort of difference). I stayed with Lubuntu because the look was more uniform to me (simpler life thing).
Mac, is a BSD based OS, so you might also consider something like FreeBSD (probably more of a learning curve, because Mac trys to take that out of the picture).
Unetbootin is a simple program that is commonly referenced, when talking about putting an ISO (bootable distro image), on a USB key.
http://unetbootin.sourceforge.net/
It is multiplatform. *run it on your old system, or your Mac* If you write it to a USB key with this, then you get the same thing every time you boot. (like a DVD) I tended to write the ISO to one drive and install to another USB key, rather then the hard drive. To me the hard drive was basically storage (kept Windows for once every couple years sort of thing). If you stick to drive that are less then your hard drive, it makes it easier to notice, so you don't overwrite your old Windows stuff, if you don't want to.
I am now looking to go more old school with my new pc system (first higher end one in a LONG time). I built an I7s, 16gb ram, 320 on two ssd's and 2gb on a hard drive, and picked up a 4gb for my current system to act as the storage server. (rarely used the netbooks now) My goal is to go more old school and see if I can learn/relearn as there is a lot I haven't done since the Redhat 5.3 days (98-99?), so I am going Slackware.
My father prefers Raspbian and Puppy Linux. But these are names for distro's that use the Linux Kernel (brains), with other packages. Distrowatch is a place to see lots of ones that might work for you. (Mint gets high recommendations) I sent him a question asking for his suggestions. (one would be don't get around static before playing with these things, he killed two in the last couple of weeks)
I am sure more later.