Besides the actual guitar, what are my options as far as hardware is concerned for connecting it to my Mac Pro? Is it possible to plug the guitar directly into the computer? Is it significantly better to run through some kind of mixer and if so what kind of connections are required?
what are my options as far as hardware is concerned for connecting it to my Mac Pro?
There are actually a large number of products out there for interfacing instruments with your Mac. I'm a big fan of Apogee gear and the Apogee One is an excellent solution for a high quality audio interface for a Mac that's super stable and sounds great. The advantage of buying something like the One is you get a high quality microphone preamp, a high impedance direct instrument input and a great AD/DA coupled with rock solid drivers.
Is it possible to plug the guitar directly into the computer?
Yes! It is. You can buy a 1/4" mono to 1/8" (also called a 3.55 mm) adapter and plug your guitar in to the adapter and the 3.55 mm end of the adapter in to the line/mic input on your Mac. The impedance on the input may not be that high (I forgot how Apple sets their input impedance on Mac Pros) so you may find the guitar signal to be really quiet. But for a basic, get-going-quick type connection to use GarageBand this works just fine.
Is it significantly better to run through some kind of mixer and if so what kind of connections are required?
It is definitely better to run through an interface that is designed for instrument-level signals. Something like the Apogee One is a solid choice but there's a myriad of solutions out there for any budget. From the low end, $50 range, right up on up to the >$5k range.
for reference and to build upon Ian's answer (I don't have the
comment perk yet), I was using direct-in (mic) on a homebuilt PC with a realtek onboard chipset. As far as I know, mac pro also uses the realtek chipset.
Once you are hooked up, you will want to look into amp simulators. There are lots of free options which will work in garage band. They are typically called VST's or AU's
there is another solution, that works great headphone portable guitar amps such as Amplug series from vox. You'll get much better results than plugging your guitar in directly, plus you can play your electric guitar anywhere with them and have control over how much volume you feed into your computer input.You will need a minijack-minijack chord.they are just amazing.Price is somewhere around 50 €