I don't understand why the speedlight would affect the useable ISO? Anyway, I happily go to ISO 3200 on the D700, and ISO 6400 in a pinch but I notice the noise starting to creep in - not a lot of noise but these days I'm spoiled!
In truth though, only you can decide what noise level you will tolerate, we all have differnt expecations and thresholds. Once you own the kit, playing is free, so crank up a simple test and see your limits for yourself
Just shot my Brother in laws wedding with D700 and SB-900 in very dark settings and shot 1600 to 3200 and no noise at 1600, very little noise at 3200. At 3200 I felt the little noise was so slight that I did not do any noise reduction.
I have only had the D700 for past three months and it continues to amaze with the noise fre images at 1600 and 3200 iso.
Well I can honestly say I happily go up to 6400 as long as you nail the exposure, in print I am still amazed how the files look-they are fantastic,plenty of detail-especially if you like the look of film.The D700 sensor produces wonderfully velvet-like grain/noise@high iso which imo I favour-it's got real charachter.BTW I don't use flash.