You can never win by getting the latest the greatest, but you can win by waiting for today's latest and greatest to drop down in price to be tommorow's midrange. True, you can always say, "wait wait wait, something better will come out, wait" then you'll never purchase anything, that's not the point, the point is, waiting to save yourself a good buck, or whatever you'd be paying now for a 7800GT, you could get something considerably better(that's a dangerous mindset, but for this case is reasonable since theres a ton of things that will be soon released)...it's just timing.
Basic trend: new stuff gets released at a premium price, old stuff slowly begins to fall, then new stuff drops in price as product matures. Right now, we are at the end of a plateau that's about to shoot back up in performance, the 7800GTX has been king for long enough, if this was 3 months ago, then things would be different, but as of now, the tables are about to turn as a new "champion" is about to arrive.