Can I ask something? How does the GTX Titan, Titan Z, and Titan Black compare to the GTX 780 and 980?
I'm not as up as some on the 980s. But here's the basic run down on the others...
Compared to the Titan Blacks, the regular Titans are the older version of the chip/card, clocked a little lower and with fewer CUDA cores, but same amount of on-board memory (6GB), Titan Zs are the same as two Titan Blacks on one card, only clocked a little lower overall and only using one PCIe 16x gen. 3 slot, so less overall performance potential, and the 780 Tis have much lower double precision (DP) compute performance (not an issue for gaming at all), and lower on-board memory.
The 780 Tis, or possibly the 980s are the way to go for gaming and likely also most other applications, especially when cost effectiveness is taken into account.
I personally do very much make use of the higher memory of the Titan Blacks for crunching multiple work units simultaneously for BOINC while sometimes also gaming on the side. The DP could come in handy if I decide to ever prioritize one of the BOINC projects I crunch for that can make use of it (MilkyWay@Home).
When the Titan Zs first came out, they were a horrible option for most people's needs as they offered lower performance in every way than two Titan Blacks, with a price tag of $1000 (USD) more than two Titan Blacks. They have since come to their senses and lowered the price for Titan Zs, but that was likely implemented to sell off their remaining stock, so you're unlikely to find them at that more reasonable price.
Edit:
After looking up some more info on the 980s, it looks like they offer less memory than Titan Blacks (4GB compared to 6GB), but have a higher clock, and use the new (more power efficient) Maxwell architecture. I think most gaming type benchmarks put them on par with the 780 Tis and the Titan Blacks, but with a slight advantage in some as well.