After the 7800GT supposed to come August 11th, I believe there will then be a 7800 (no suffix).
If the 7800GT is guestimated to cost around $449, then the 7800 would problably cost anywhere in the $300 range.
During all this time, though, I think prices of the current Geforce 6 series would drop, and a 6800 GT costing around $350 could decrease to a price of your liking.
I think that when the r520 comes out, nVidia will then release it's full line up of cards, with the exception of any card higher than the GTX (like the Ultra or such.)
Will there be a 7600 priced less than $200? I don't know, but I would probably doubt it would be priced that low anytime soon, because if it was any better than a 6800 Ultra, then the 6800 Ultra card would be moved down in price, and I don't think we'll be seeing a 6800 Ultra under $200 anytime soon.
For now, a 6600GT would probably be alright, but if you could wait, then maybe the 6800 GT would decrease in price enough for your liking. But I don't see it likely that the 7000 series releasing low -mid range cards would cost lower than $200, anytime soon, atleast.
I mean, I guess theortically speaking, if the 7800 was in the $300 range, then the 7600 could be in the $200 range. But the question is when this card will come. Honestly, if I were you, I'd wait it out. I wouldn't expect nVidia to hold out a long time just to deliver the rest of the series. Like I said, with the release of the r520, there will be more competition, and a stronger chance nVidia releases it's low -mid ranged cards.
Keep in mind, nothing I am saying is really confirmed or such. It's just my educated guess. Some others may argue that, actually, my guesses aren't educated at all, and hopefully they'll be able to give their opinions as well, to better help you make a decision.