Where is my hardware bottleneck? *5 Boxing on single PC*

[Thulos]

It's not obvious because it is possible that textures are only cached one time instead of caching the same textures 5x times.

Short: it is not possible.
Long: textures can be cached as long as some app knows that they come from same source. That's why symlinking works: the OS file cache manager knows that 5 apps referencing their data files are actually referencing the same file and gives them cached data instead of reading disk over and over again. But when app uses D3D interfaces to render data, there are no "texture files" at this point. There are some binary structures and neither OS video driver nor D3D manager know their source, they only know that some app (or 5x apps) asked to render some data.
WDDM1.1 changes it a bit: application can tell video driver that it wants to render multiple instances of the same object. So in this case you can actually save some memory, because video driver won't allocate it for all instances. But again, it works only inside one app and it requires WDDM1.1 drivers which are only available in beta version under Windows 7 for now.

Thanks for this explanation, this does shoot down the shared texture memory thing. I still would like to have some way to validate your claims. I may load into my XP 64 instance and get it up to date to 5 box. I wish Vista would give us a way to check video memory usage. The only nvidia card I know of that gives us more than 1gig of memory is the very expensive gtx 295.