I would go for the body first. The 24-70 is won't give you increased flexibility. You will still probably use your 50 1.8 more due to weight and speed and still reach for your 14-24 for wide. No question, I owned the 24-70 for almost 6 months and never used it on my D7000 bodies as it is BLOODY heavy (or my fast primes offered better low light performance). I have only recently used it for my new D800E and mostly when I am shooting studio work. My lens of choice for general purpose is usually my Sigma 50 F1.4 .
Your money spent here will gain you almost no utility if the lens won't be used. This lens amazing and is one of the few that will won't be out resolved by the D800. However, it takes the D800 to give it it's true potential. The 24-70 range on DX is also not a ideal zoom range for general purpose use... not wide enough (oh yeah, I have to mention again... too freaking big and heavy). I assure you that you will run back to your 50 F1.8 if that's what you have been using for your current work, or the 14-24 which you use for landscapes now.
The D90 is already fairly old tech. If you were on a D7000, there could be an argument for hanging on to that. The D7000 represents an incredible leap over the D90 and my previous D300. I could not believe the advances in ISO performance it gave. In addition, there are 51 focus points and a myriad of other improvements.... The D800E I have now is again another leap...
Buy the latest if you can afford it. Contrary to what others may be saying, the D700 did not drop in price much (or at all) over the 2-3 years that it was out. Only now has the price dropped with the D800 out. You can wait but people who buy the latest and update when the following model comes only loses about 15-25% in resale value. Try holding on to a D700 for another 3 years and see how much people are willing to pay then (or if you can find buyers). I don't see much interest in D200's or D2x's these days. D90's are gonna be hard to sell with D3200 offering so much more performance for so little! If you bought the D90 new... you will be losing a fair bit more in terms of percentage depreciation by the year if you sell it today.
My point in summary... the D800 is going to offer you SO MUCH MORE in terms of gear satisfaction and image delivering performance than the 24-70 will do on your D90 now..... this is with your current lenses. (You may want to upgrade to the 50 1.4 or even the 85 F1.8). The camera you buy won't drop much in price for the next 3 years... You are going to get the 24-70 anyway.... but getting it now isn't going to add much to your fun... getting it later will make you appreciate your D800 more
If the camera body is meant to be upgraded and sold in time (as opposed to lenses), isn't it better to get it early in the model's life cycle so it maintains it's value longer?