Re Flashboards. Yes, there are several options. Small=516MB - Medium=1GB & Large=2GB. Only place I've seen the the 2 gig version is at Motifator.com. Prices vary . Far as I can tell, only Motifator has the 2 GB version at $549. Amazon has the 1BG for 249, as do other sites. Depending on how much flash storage you think you might need, remember you only have ONE slot. AT this point I'm not clear on how many voices (or actually how much useable data) you can put on the external flash drive, or how you might swap voices from your computer, but USB flash drives and/or onboard computer libraries should open up many possibilities. Good luck with these.

If you're looking for more (and maybe better, and certainly different) piano voices, check out GospelMusicians.com -- several demos on youtube. Some absolutely amazing piano voices there. I bought Jamal's Flash collection. Well worth the investment.

1) Can I substitute a different voice ... say trade the sax for piano ... within the arp ? If not can I somehow extract the drum part only and use as a part in my sequenced song ?

You are very confused. A VOICE = an instrument sound. For example, a piano VOICE is "Full Concert Grand.
An ARP = a musical phrase that can be assigned to any Voice. In a Performance you can assign as many as four VOICES, one to each PART.

You can assign arpeggio phrases to play the Parts. We highly recommend you spend time with the Quick Guide portion of the Manuals to get an idea of the terminology and what can be accomplished.

2) Should I get a the external Flashboard ? Is there more than one ? I can see that I will want more voices to add to the onboard collection.

An external Flash board will allow you to add new Waveforms and Samples to your MOXF arsenal.

3) I'm wondering why the piano voices sound so bad coming out of my Yamaha speakers. The sound through the headphones is so much better. Do I need to tweak the external speakers somehow in the utility mode
?

This is something you are doing wrong, and should fix. The signal at the headphones is always (or should be) the same as what comes out the main left and right outputs. If the sound coming out of your speakers is "so bad" then it begs the question: "How do you have things setup?"

4) I am still trying to understand how to save patterns , songs , performances and where they go when I save them.

Again, you'll find the Manuals absolutely excellent at not only how to save that data, but at helping you understand exactly how it all works.