Reg,
WOW.

I guess my first question would be, do you like FM Synthesis? In other words, is there some particular reason you want to keep the Yamaha keyboards? Since most new sounds are ‘sample/playback’, like the D110 and SC50, and the Yamahas are FM synthesis, it seems to me that you would want to upgrade some something newer.

Next comes the G5. Depending on the exact model you have, you can probably use M-Audio sound cards. For the number of modules and keyboards you have, I think the M-Audio Delta 1010LT would be a perfect solution for you.
http://www.m-audio.com/products/en_us/Delta1010LT.html
This will eliminate the need for the mixer.

Hard drives. You will need two physical hard drives in your computer, one for the ‘system’ drive, and where you have Band In A Box and whatever DAW program you’re going to use, and then one where you’ll store the audio. You should always use a second drive for storing audio, dedicated to just that.

Software: To record from multiple sources, i.e., two keyboards and two modules, simultaneously, you’ll need to have a program that will allow you multiple audio inputs. As far as Mac goes, I am not in a position to offer suggestions, but Cubase may be one.

Band In A Box will give you a backing track, but only supports one MIDI output and one Audio interface. So, using BIAB as the main application for recording music isn’t that great.
There is Apple Logic Express 8
http://www.sweetwater.com/store/detail/LogicExp8/
This is an entry level application for recording audio, and will also give you several software synths along with it, as ‘plug ins.’

You can try Cubase Essential 4
http://www.sweetwater.com/store/detail/Cubase4Ess/
Again, entry level for the Mac.

You could spend a little more and go with Cubase 5 Studio, which has unlimited MIDI and audio tracks
http://www.sweetwater.com/store/detail/Cubase5Stu/

Finally, you’ll need a MIDI interface. The M-Audio MIDISport 4x4 might be a good idea. I’m sure that the G5 has at least one USB output on it, and you can plug this interface into it, and then direct the outputs from Cubase, or Logic or whatever to the separate midi ports on this.

Finally, I would suggest looking at replacing one or all of your sound devices with something newer. Keeping one of the Yamahas to use as a controller keyboard is okay, but I wouldn’t want to use them for sound generation, unless you really *do* want that FM synthesis sound.
I would recommend listening to the Ketron SD2, and considering replacing the SC50 with this: http://www.pgmusic.com/ketronsd2.htm
If you need help deciding, listen to these demos of the SD2. FYI, the SD2, SD4 and SD5 all use the same sound engine, so you can listen interchangeably and still get the same sound ideas.

Finally, those who use Macs are waiting for the latest release of BIAB that brings the compatibility to that of the PC users. Peter has been promising, but I have no idea when it will happen. That will give you Real Instruments and Real Drums, which will also be a good thing, and you should plan on that upgrade as well.

Welcome to the world of computer audio. We’ll be around to help you.

Gary

I'm blessed watching God do what He does best. I've had a few rough years, and I'm still not back to where I want to be, but I'm on the way and things are looking far better now than what they were!