"So gpu is not actually 'only' 256 or 'up to' 512 but somewhere in between (if implemented)."
More important is that you can directly write to GPU memory space from storage in the console space. In Windows you have to write your textures to system ram, then over to the GPU's ram pool. In effect, anything that you have in GPU memory you also want to keep duplicated in system memory so that if it is flushed from the GPU you don't have to go all the way back to storage for the texture data. This makes direct console-to-Windows comparisons silly. 256+256MB in the console space is roughly equivalent to 512+256MB in PC terms, simply due to the lack of wasted duplication.
Things get much sillier when you look at a unified console memory structure versus a Windows unified memory structure. The 360's flat chunk of 512MB that anything can access means textures write to ram once. In Windows with something like an AMD APU or Intel with integrated graphics, video memory comes out of system memory. Texture duplication is still required though, as you are not allowed to directly write texture data to video ram from storage. So, a 512MB system, with 256M shared video memory ends up with duplicate textures in each half of that split 512MB pool. The result is much greater amounts of wasted space.
On a Windows XP system (these consoles' contemporary), to roughly match the 512MB that the PS3 and 360 shipped with you need: 512MB on your video card. The same 512MB to match for wasted texture storage. Plus 512/256 to match for system storage for 360/PS3. Also 256MB for Windows XP. In other words, you need 1.25GB of system ram with a 512MB video card to match what the 360 does with it's much smaller 512MB unified pool. To match a PS3 you need about 1GB system ram, plus a 512MB video card.
Extrapolate that to 8GB of unified memory for a new console like the PS4. You are now looking at the equivalent of an 8GB video card paired up with about 20GB of ram to account for duplicate textures, full system ram, and the 4GB that modern Windows wants in order to turn on
Responding again to "And 512mb makes it ANY better?"
The 512MB pool on the 360 is in no way restricting whether the 360 hits 1080p. That is much more of a bandwidth issue. It in no way holds back the 360 from 60fps. That is more of a GPU age issue. The worst thing you can point at and say "512MB causes this!" is texture pop-in. Either all textures needed could fit and be lower resolution, or sharper textures can be used but loaded at the last possible moment. That is ram size in play.
The story here in relation to new consoles is not ram size. The story is exactly what I tweeted the day of the PS4 announcement, here now in censored form: "No way. Sony finally got it through their thick frakking heads and are using a unified memory architecture on a console!?! Took long enough."
More important is that you can directly write to GPU memory space from storage in the console space. In Windows you have to write your textures to system ram, then over to the GPU's ram pool. In effect, anything that you have in GPU memory you also want to keep duplicated in system memory so that if it is flushed from the GPU you don't have to go all the way back to storage for the texture data. This makes direct console-to-Windows comparisons silly. 256+256MB in the console space is roughly equivalent to 512+256MB in PC terms, simply due to the lack of wasted duplication.
Things get much sillier when you look at a unified console memory structure versus a Windows unified memory structure. The 360's flat chunk of 512MB that anything can access means textures write to ram once. In Windows with something like an AMD APU or Intel with integrated graphics, video memory comes out of system memory. Texture duplication is still required though, as you are not allowed to directly write texture data to video ram from storage. So, a 512MB system, with 256M shared video memory ends up with duplicate textures in each half of that split 512MB pool. The result is much greater amounts of wasted space.
On a Windows XP system (these consoles' contemporary), to roughly match the 512MB that the PS3 and 360 shipped with you need: 512MB on your video card. The same 512MB to match for wasted texture storage. Plus 512/256 to match for system storage for 360/PS3. Also 256MB for Windows XP. In other words, you need 1.25GB of system ram with a 512MB video card to match what the 360 does with it's much smaller 512MB unified pool. To match a PS3 you need about 1GB system ram, plus a 512MB video card.
Extrapolate that to 8GB of unified memory for a new console like the PS4. You are now looking at the equivalent of an 8GB video card paired up with about 20GB of ram to account for duplicate textures, full system ram, and the 4GB that modern Windows wants in order to turn on
Responding again to "And 512mb makes it ANY better?"
The 512MB pool on the 360 is in no way restricting whether the 360 hits 1080p. That is much more of a bandwidth issue. It in no way holds back the 360 from 60fps. That is more of a GPU age issue. The worst thing you can point at and say "512MB causes this!" is texture pop-in. Either all textures needed could fit and be lower resolution, or sharper textures can be used but loaded at the last possible moment. That is ram size in play.
The story here in relation to new consoles is not ram size. The story is exactly what I tweeted the day of the PS4 announcement, here now in censored form: "No way. Sony finally got it through their thick frakking heads and are using a unified memory architecture on a console!?! Took long enough."



















Again, this is no different from 2006 when depth-of-field showed up in Tomb Raider Legend, ran great on the 360 and choked every one of our video cards. Or when Oblivion came out, ran just fine on the 360 but ran like crap on technically "superior" PC hardware without some serious ini tweaking.