I read that full frame IQ is better, hence my decision to step up from a smaller sensor. I want to invest in some good FX glass (24-70mm for starters probably) but am having trouble justifying upgrading to D700, which is the same 12MP as my D300. I will probably wait for the D700x (or whatever they call it) and invest in lenses now. I recently acquired a 70-180 Nikkor macro zoom that I am really liking. I still have an 18-200mm though but I try to avoid that range due to IQ concerns. Mainly using a Tokina 12-24mm, which gives good results but is not always that reliable.
Should I just be investing in good Nikkor glass, then, and wait for the D700x?
A few more thoughts...
There always is something better out there, and there always will be. The bigger question is at what point do we hop off the constant upgrade train, or, perhaps, where should we be concentrating our energy when we do upgrade, and also, if we have really maximized everything our current camera is capable of doing. My own situation is similar to yours I think - I own D2X and D300 bodies and while I considered and evaluated a D3, because I only shoot at low ISO I didn't see any advantage to the D700/D3. Of all the bodies Nikon makes, for what I do personally (which is studio and some landscape), the D3X would be the best, and the D2X would be my second choice, with the D300 and D700 or D3 coming in closely in third place (in the Nikon line). Of course if I were a wedding and event shooter where I needed a lot of middle and high ISO, my answer would be completely different. For what I do I found that building a very high quality lens collection first made more sense, and that's the approach I have taken. Bodies will come and go and there will always be a new toy that everyone in the forums will be talking about, compelling many a user to feel they need to upgrade long before they have ever maximized what the "old" camera is capable of, but excellent glass has longer staying power. Incidentally, my reason for staying Nikon personally is that in the focal length ranges I typically use, neither Canon nor Sony has what I prefer - even if I won the powerball tomorrow, I would not change brands simply because of this; however, someone else might have different focal length needs and then Canon or Sony might be the better option - it is not always so clearly stated that one is better than the other in absolute terms - depends on the ranges you need as each manufacturer has strong lenses, just not across the board.
You've picked up a very desirable lens in that 70-180 macro - I've never heard a bad thing about it. If you are happy with the Nikon ergonomics and after an honest talk with yourself feel you can wait, I really can't see why you would need to change systems. On the flip side, if you have a client situation that is demanding a higher resolution file and you need to produce this right now in order to get some much needed business, than certainly you have to make a move. And while I absolutely loathe anything Minolta ever did and in a 30 year span in photography never found their gear reliable, I'd also have to honestly suggest taking a long look at the Sony A900 since I believe their glass, particularly in the Sony/Zeiss offerings, while not offering as broad a set of choices compared to Nikon and Canon, is a step ahead of where Canon is at currently in terms of lens quality at a few popular focal length ranges.
My best "guess", not knowing every aspect of your situation, is that you wait for the D700x and slowly build a lens collection, unless you need high ISO capability today, in which case it would be worth it to pick up a D700 (and then pair it with a D700x when it comes out). In the mean time, peruse the writings of folks like Thom Hogan in the dpreview forums and at his website and you'll see he often mentions that the differences between the D3X and D3 are not always clear, dependent on the lens, subject matter, support, and so forth, and that other times it's quite clear. The more I read, the more I see that just buying the higher resolution body is only part of the overall package in terms of being able to produce larger, better prints.
Good luck with your choice and don't let the equipment rule you, rule the equipment !
-mike