justanoob :
I just started play GTA V and I have 2 r9 270x's that have 2 GB's of ram each. In GTA V however it shows I have 4 GB's and last time I checked CS/SLI doesn't combine vram( until dx 12 apparently). After playing for a bit(story mode) the game crashed and said i was low on memory. So is GTA V combining Vram or is it stealing RAM.
The dedicated memory on each graphics card are independently addressable. If you have two GPUs with 2GiB of GDDR5 each, this will be managed as two separate blocks of 2GiB for a total of 4GiB.
However, with the way automated resource management is implemented in DirectX and WDDM, most memory is mirrored across all adapters when SLI or CrossfireX are used.
For the purposes of accounting, each adapter's memory is discrete. Each adapter will have a copy of each buffer, shader, etc... so enabling certain memory intensive graphical options will have twice the impact on a 2-way Crossfire/SLI configuration as it would on a single GPU configuration.