I typically prefer to shoot with off camera flash, but ISO 100 is not really an option - although I could pick up a couple of more flashes and it might be possible...and it may be cheaper than upgrading to a 700.
My technique is good, I shoot RAW and the images overall are superb.
I think I may just be splitting hairs on this and I'm starting to think that there won't be enough of an increase in IQ to make a difference.
Although I prefer flash to HDR, I have tried it some with RAW images creating multiple files, but it introduces more noise than it would by actually taking 7-9 photos. This was really a side point.
It doesn't sound like there would be much IQ difference by switching to a 700, but I may rent one to see. Just to see what you think, here is a link to a photo I took last week when my daughter was home sick with me and I was playing around. I actually tried this shot at ISO 100, but I didn't have enough flash power so I had to lift it in post. I started in Capture and finished in photoshop. I have Lightroom, but I can't ever seem to get results that I like as much as using Capture. This is a full size jpeg with to SB-600's camera left and right bounced backwards to reflect back on the scene. I'm not at my computer with my actual files, I just happened to have this one in Dropbox so I uploaded it to one of my websites. I exposed for the light on the right side and brought as much flash power as I had. I could have opened the aperture up a little, but not too much more. If you view it large, you will see the noise. A little of it may be sharpening, but it's got a good bit of noise. Look in particular at the wood shelf under the TV and no, the image on the screen is not masked in.
http:// http://www.ryanwardenterprises DOT com/test-100-iso.jpg (not sure about links here so I separated it....)