Re: No Mic input just speaker &...
agreed, that is a nice rig!
As for sound cards and whatnot, if you are not really that bothered about wanting to use mics and ONLY want to record your guitar 'dry' as it were, i.e. not through a amp using only the DI output from the guitar itself, then as long as the USB 1/4 inch convertor you have is designed to work with instruments, what you have already is fine.
The headphone output on your mac pro will work to plug straight in to your denon amp by using a 3.5mm to two RCA plug lead.
Your denon just has to have a spare input and that can be used for connecting the output from your computer.
You can look at getting an M-Audio, Edirol, MBox, Apogee solo delete where appropriate sound card, but, I don't want you to spend your money unnecessarily... unless you don't mind!
It all depends on what you want to use it for and how you want to play around.
There are lots of different brands of software amp simulation models outside of what is in GarageBand. Logic is GarageBand's bigger brother and that has different amp plugins, it's not amazingly better to be honest but there is one from a company called Native Instruments called Guitar Rig that's popular. There's another one called Amplitube, Waves do one called GTR I think it is, Audio Ease have lots of different speakers modelled in Speakerphone 2 and have just release a FREE piece of software called Cabinet which has a few cabinets you can use on your guitar.
It really depends on how you see yourself experimenting. If you have a few real guitar amplifiers and you see yourself wanting to send signals out from your computer through the guitar amps and recording the result then you would be better off with a sound card other than what's in your computer.
If you only want to work with software, then you can use the headphone output on your computer along with the 1/4 inch to USB cable (if it's designed to work with Guitars).... however, just bear in mind that like most things in life, if you are prepared to spend a bit of money on something, you can improve the sound quality of even this scenario by getting a dedicated sound card.