A lot depends on whether you want to gig with it, or have it at home...
On a gig, built in speakers have little use unless you do super quiet restaurant gigs, so you'll need that amp no matter what you get (or a PA).
At home, depending on where you set up, if you have a reasonably beefy home stereo, that might be all you need (go into it's aux in jacks). Quite a few use the Logitech computer speaker 2.1 system, or something similar if you need a home system that's affordable but with a bit of poke.
Others (like myself) use near-field studio monitors (Mackie HR824's for me) for the ultimate in accuracy and clarity (balance your registrations on these, they'll sound good on anything!).
But bear in mind, you're about to make a $3000-4000 arranger purchase (if you get G70/E80). You REALLY want to hear that through a $300 audio system??
The question that I would be asking, to make my decision easier is... 'Do I play piano?' If you do (or wish to learn), the 61 keys of the E80/X6/G6 are just not adequate (and a workstation is NO substitute for an arranger).
I would chose the G70. It's sound quality (actually, all those choices are about equal) is great, it's OS is not TOO different from the EM2k (X6 and G6 are totally different beasts), and right now you can find them going for considerably less than they were 3 years ago.
Sure, it's not the latest, greatest, but the money you'll save compared to an E80 will allow you to buy some decent monitors or a small PA.
Bottom line though is you HAVE to go and listen for yourself... Only YOU can decide if sound quality is good enough FOR YOU...
[This message has been edited by Diki (edited 02-21-2008).]