Tom, I do exactly what it is you're wanting to do in my Gig-Rig, which consists of the following:
Roland Fantom G7
Yamaha Motif XS6
Yamaha Motif XS Rack
Roland Fantom XR (Rack) with 6 SRX boards
Korg Triton Rack with 8 EXB boards
Roland AX-7 Keytar
Korg R3 Synth/Vocoder
Here's how I do it:
The G7 is the master controller. Running from Live Mode, you will be able to create all your splits/layers of internal Fantom G sounds, PLUS all your external sounds via MIDI. The Triton Rack runs on MIDI Channel 1 (because I used to gig with a Triton Extreme and downsized to just the rack plus 2 keyboards...it was too much to haul). The Motif XS6 runs on Ch 2, the XR on 3, the Rack XS on 4, and the R3 on 5. By progamming in all your MSB/LSB settings (on the Motif Rack XS, it's 63 for the MSB, the LSB varies depending on what bank of presets you want), and splitting/layering up the keyboard just the way you want it, you can then save the entire "combination" (in Korg-Speak) or "Performance" (in Yamaha/Roland-Speak) as a Live Set. From then on, it's simply a matter of dialing in that Live Set and your Rack will automatically switch to whatever patches it is you want and you're cooking. Can't be beat! Keep in mind that I don't use the sequencer yet (I'm learning to use it now), so once I implement that into my workflow things may change dramatically.
And yes, the rumors (and liklihood) of Yamaha launching a new Motif this year are strong. Motif was announced at NAMM of 07 and was in stores by March 07, right along with the Korg M3. Yamaha in the past has released a new Motif every 3 years, so this year is expected to be the year of the new one...and I've read that the name will be called the "Motif GS" but don't quote me on that. In fact, a posting in a thread on this subject popped up over on Motifator.com (Yammy's customer-service center) about 2 months ago with a SUPPOSED "picture" of this new Motif GS and it's incredible (if true). It showed a huge screen very similar to the FG's (and supposedly without the glare of the XS), and was rumored to have 160 note polyphony, 3x the wave ROM/internal user banks, new synth AND weighted keybeds for each model, velocity pads, and some other stuff I don't remember. The thread was taken down within 20 minutes of its posting...so who knows if it's true or not. I would highly expect us to see a new Motif in 2 months...and I'm sure it will give the FG a run for its money.
Regardless, the FG is absolutely AMAZING, and it's unquestionably THE easiest synth I've ever used...I'm so spoiled by it that even the Yamaha reps I know and talk to agree.

I'm confident you'll fall in love with your G7 as soon as you get it!