The only reason to buy a Titan X would be a need for 12gb of vram.
Today, I see nothing that would justify that.
VRAM has become a marketing issue.
My understanding is that vram is more of a performance issue than a functional issue.
A game needs to have most of the data in vram that it uses most of the time.
Somewhat like real ram.
If a game needs something not in vram, it needs to get it across the pcie boundary
hopefully from real ram and hopefully not from a hard drive.
It is not informative to know to what level the available vram is filled.
Possibly much of what is there is not needed.
What is not known is the rate of vram exchange.
Vram is managed by the Graphics card driver, so there may be differences in effectiveness between amd and nvidia cards.
Here is an older performance test comparing 2gb with 4gb vram.
http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/
Spoiler... not a significant difference.
Are you planning on 4k gaming or triple monitor gaming?
If so, even the best GTX980ti hybrid is not likely to be enough and you should plan on dual cards.
Think carefully about your case. There are usually not enough openings for two or 3 sets of radiators for dual cards and a liquid cpu cooler.
If you are looking at high end gaming, I think I would wait for Skylake I7-6700K.
It is likely to have the fastest cores around.
X99 lets you install 6 and 8 core cpus, but few games can use more than 2-3 cores.
To prove this to yourself on your current FX-8350,
experiment with removing one or more cores. You can do this in the windows msconfig boot advanced options option. set the number of processors to less than you have.
This will tell you how sensitive your games are to the benefits of many cores.