I assume your 50" TV has at least 1080 HD resolution, perhaps even 4K?
Your x8/x8 GPU SLI setup is excellent, and a great balance of price-vs-performance-vs-overkill. Although a pair of GTX780 cards is indeed overkill for playing mid-weight 3D games at 1920x1080, a single GTX780 could probably sustain 60fps running WoW with all the eye-candy turned on at that resolution.
The 780 (and 780Ti and TITAN) cards were epic in their time and they're still mighty beasts. The new TitanX card is an epic beast (lookit that phat 12GB of GDDR5!), but it's no true Titan. The 980Ti is also impressive, exactly the incremental upgrade on the 980 that people expected. Both of these cards will command the top of the price tier until NVidia releases their new product line (circa Q2/2016, people say), but NVidia will likely keep trickling out other cards to "fill out" intermediate rungs on the pricing ladder. AMD is (or will soon be) offering some new "ultra-enthusiast" toys with phenomenal specs as well. Expect GPU card prices from both manufacturers to basically sit where they are (and maybe inflate a little, as supplies are brought down a bit) until their new rollouts push existing cards down the price hierarchy and force a little more competition. Remaining stocks of "obsolete" GPUs (GTX 660Ti, HD79xx, etc) are sometimes found at awesome liquidation prices these days, definitely worth scoring if your budget is tight and/or extreme performance from a couple years ago is adequate for your needs.
NV Partners (including Asus) probably have more detailed access to NVidia's product schedules (after all, they will already be bidding for GPUs in volume so they can start in-house binning processes and, ultimately, offer a whole range of factory-overclocked card models). But they're probably locked down with heavy NDA shackles, NVidia has legally (and economically) sanctioned violators who've leaked before.
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GTX780 chip supports G-Sync. NVidia built it into the GPU design (or at least they built it into NV Partner licensing, lol), it's not an optional add-on, and it would be kinda stupid for any manufacturer to sacrifice such a major selling point in return for pennies anyhow. Monitors which support G-Sync carry a hefty price premium, partly imposed by the NVidia license.
I personally think the best bang for the buck, these days, would be 4GB GTX670/680 and 3GB R9-280/280X cards at the low end and 4GB GTX980 or R9-290X cards at the high end. Yes there are cheaper cards, and yes there are better cards, but these models offer (I think) the best overall compromise for most gamers and most gamers' budgets.
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