Video cards have long had more than 512MB VRAM so again x64 is needed to be fully exploit the card. VRAM is shadowed in main memory. The DMA controller copies the assembled frame while the CPU works on the next one. With the GTX Titan X 12GB, at least 32GB of memory is needed to be able to leverage the card properly. GTX Titan X 12GB SLI would be best off with 64GB of main memory.
When running 32-bit games with 64-bit Windows, they can allocate more more memory. Left 4 Dead 2 can allocate 1024MB VRAM with 64-bit Windows without problems .The performance gains are best seen with native 64-bit games like Wolfenstein: The Old Blood which can take advantage of 4GB and 8GB video cards.
The 64-bit version of Rage ran much better with our old GTX 260 SLI. Rage is designed for dual video cards and it took full advantage. Grand Theft Auto V and Battlefield: Hardline work best with 4096MB video cards in CFX or SLI configuration.
Our old HD 6970 has a 256-bit bus while our more recent GTX 660 Ti has a 192-bit memory bus. The R9 Fury X has 4096-bit memory for increased bandwidth. Lower cost cards may have a 64-bit or 128-bit VRAM system.
MEMORY LIMITS AND VIDEO CARDS
Devices (video cards) have to map their memory below 4 GB for compatibility with non physical address extension (PAE) aware Windows releases. Therefore, if the system has 4GB of RAM, some of it is either disabled or is remapped above 4GB by the BIOS. If the memory is remapped, X64 Windows can use this memory. X86 client versions of Windows don’t support physical memory above the 4GB mark, so they can’t access these remapped regions. Any 64-bit Windows or x86 Server release can.
X86 client versions with PAE enabled do have a usable 37-bit (128 GB) physical address space. The limit that these versions impose is the highest permitted physical RAM address, not the size of the IO space. That means PAE-aware drivers can actually use physical space above 4 GB if they want. For example, drivers could map the “lost” memory regions located above 4 GB and expose this memory as a RAM disk.
Clearly using 64-bit Windows gets around the headaches of PAE workarounds. By simply have 64-bits of usable address space, PAE and native 64-bit games will all work fine.
Enabling AA in Battlefield 4 will increase the VRAM needed by around 20%.
Far Cry 4 actually doesn’t perform too badly at all at 1080p, but with AA enabled you’re going to need 3GB+.
The Witcher 3, HBAO+ doesn’t do too much at all to VRAM consumption.
Shadow of Mordor is much more demanding for VRAM, It needs a 4GB card already and with AA enabled it needs 4.7GB which means 8GB cards are needed.
Battlefield 4 jumps up from 1.9GB to nearly 2.3GB at 1440p with AA applied.
Metro: Last Light uses another 200MB+ of VRAM at 1440p with AA enabled.
Far Cry 4 really starts needing lots of VRAM and 4GB cards can barely contain it.
Far Cry 4 uses a huge 5.7GB at 4K with AA enabled, a large jump from the 3.7GB without AA enabled. This is why 8GB video cards are available. They are needed!
Ambient Occlusion enabled in The Witcher 3 sees the game using another 200MB of VRAM.
Battlefield 4 uses nearly 3GB of VRAM with AA enabled versus 2.3GB without AA.
Metro: Last Light consumes just over 2GB of VRAM with AA enabled, versus the 1.6GB used without AA.