With the D800 we're already hitting diffraction around F/9 and need lenses that give us 2456 LP/PH (about 100 LP/MM) or higher resolution.
The Nikon 24-70 F/2.8 exceeds this in almost every focal length even wide open (extreme corners at 40mm being where it falls only a tad below). So you should see quite a difference between a D700 and a D800 image shot with this lens (all other things being equal).
However, the D800 simply has more resolution than a Nikon 28-300 F/3.5-5.6 can deliver in some configurations, even when stopped down. At 28mm you'll need to go to F/5.6 to bring the borders in line and then to F/8 to bring the extreme borders in line. Don't misunderstand - you'll still see more detail in the D800 versus D700 shot since the D700 isn't capturing all of the information the lens can deliver, but the D800 sensor is simply out-performing the lens at times, whereas the D700 sensor does not, so a different lens ( e.g. the 24-70 ) will produce sharper results over the 28-300 in some situations (again, all other things being equal).
Basically, the D700 does let you shoot with a wide variety of Nikon and Third Party glass and get really good results, however this won't be the case when you go to the D800/D800E. You might need to do a lens arsenal upgrade to get the most out of it, not to mention a stricter shooting technique. This is the same issue we encountered with the D7000.