Andrei123 :
Titan X or just the Titan ? If it's the Titan X then go for it , 12GB of VRam will be very useful , 4GB of Vram is barely enough for 1080p ultra nowadays. Also from which GPU are you upgrading ? What PSU have you got ?
On what are you claiming that 1080p pushed 4GB VRAM? I'd be happily corrected if you have benchmarks/reviews to back you up, but I do not believe that's true at all.
Here's a article pitting CFX 290X 4GB vs 8GB at 4K.
http://www.tweaktown.com/tweakipedia/68/amd-radeon-r9-290x-4gb-vs-8gb-4k-maxed-settings/index.html
They conclude that there's very little difference and even for crossfire and 4K gaming the price difference doesn't justify the additional RAM. They did produce one benchmark where the 8GB pulled significantly ahead, but that was running 200% super-sampling, or 8K resolution... that's equivalent to 16 1080p screens (they say it's 8 1080p screens, but my understanding is that super sampling increases the resolution across two axis... so should be 16 x 1080p... but even if it's 8 times 1080p, my point stands).
If it's hard to justify at 4K, then I'd suggest going above 4GB VRAM for 1080p will offer very little for the next couple of years.
Again, I'm happy to be corrected on this if you have benchmarks or articles that suggest otherwise, but my understanding is that 4GB is at very least plenty, and in many cases overkill for 1080p gaming, ultra or otherwise.