Nvidia likes to add new cards every quarter or so however they aren't always cards that are the most powerful. The 7900GS came in full force in September and has pretty much has taken over the 7900GT's place as they don't even make the GT's anymore.
Usually it's like every half a year or so that they release a card that's more powerful than their current powerhouse which would be soon...however technology is never certain as it took decades to perfect dual-core and now we have Quad-Cores coming? The time it takes to release a new product is never certain and we can only make speculations.