Well, the best case scenario for the RAM is obviously very unlikely, but it's based on a few observations:
- Nintendo have a history of using split memory pools, and if they decide that they want to do so, but that SRAM is out of the picture, then HBM may be their only option.
- Nintendo used a very large amount of RAM relative to the system's power with Wii U, so they may do so again.
- Samsung is entering mass production of 48Gb 3733MT/s 64-bit I/O LPDDR4 chips, so they could actually get 12GB of 60GB/s main memory with only two chips. (Comparatively 12GB of DDR4 would likely be cheaper, but would currently require at least 12 chips)
- A large component of the cost of HBM isn't the chips themselves, but the cost of the interposer that the SoC and HBM sit on, the cost of assembling everything together into a package, and of course the logistical costs associated with all of these. For AMD's Fiji, the GPU die is manufactured by TSMC, the HBM by SK Hynix, the interposer by UMC and then it's all packaged together by Ankor. These costs don't vary all that much whether you're using one chip of HBM or four chips of HBM, you still need an interposer and you still need a company to assemble the final package.
As a simple example, let's consider the possibility that each 1GB stack of HBM1 costs $5, and the added costs of assembly, interposer, etc, are $20, regardless of how much HBM you use. So, for Fiji, AMD would be spending about half of the cost on chips and half on assembly. In Nintendo's case, they could get 1GB of HBM at 128GB/s for $25, or they could double both the quantity and bandwidth for a marginal 20% cost increase. Similarly they could quadruple both the quantity and capacity for just a 60% cost increase (which may seem excessive, but may allow them to push out the occasional game at 4K, which they might want to do if they're looking to compete on the graphical front again).
Alternatively, it's even possible that the packaging costs exceed the cost of the HBM chips themselves, perhaps as a result of SK Hynix giving Nintendo a good deal to ensure a continued stream of revenue from their HBM1 production line when AMD moves over to HBM2. In this case it would end up being 4GB by default, as the marginal savings from reducing it would be trivial.
That's not to say I at all expect them to come out with 4GB of HBM and 12GB of LPDDR4, but there is a not-quite-impossible set of circumstances which could theoretically end up with Nintendo going that route.
