I am a firm beliver of just get what you want now. Don't rely on what is comming out, unless it just isn't available in your region yet. Or has a FOR SURE set dates! (Seems like all the droolie cards are always out of stock.)
Look at how the 4GHz intel was stalled. Dual cores... The list just goes on. If your current needs are for a lot of moola in video, get what you can order or willing to wait on back order. I like both platforms, they kinda balance each other out in a sort of competitive way.
What are your ultimate goals with this new card? How long do you plan to use this card? Do you need new parts often? What games do you usually run? There is so many factors to ask yourself.
One interesting factor is the # of fps in some game engines. The new DOOM engine seems to thrive under the NVidia platform. On the other hand ATI does well under HL2. So if you are in the market for up and comming releases, look at what engine they might be using.
Now if your pockets never run out of money, just convert to SLI and enjoy the benifits. Later on you can upgrade and have the "joy of the build".
For now, the top ATI and NVidia offerings are both fine options, unlike this time last year. It kinda boils down to card maker and platform of your choice.
(hint: has the 6800xx fixed its video issues yet?)