Damn. Hit post by accident.
Anyway, you seem to have a lot of misconceptions with what Windows is actually doing.
First, Windows XP Mode IS a MS Virtual PC. The bonuses of this over your own XP image are programs launch-able to your Win7 desktop (outside of the Virtual PC Window) without starting up the full Virtual PC and you don't need to provide a license. Now having your own image of XP does have its own benefits as XP Mode is a scaled back version of XP just to run programs and do basic stuff.
Second, Vista works perfectly fine on 2 GB so long as you don't have an large amount of programs open that use a lot of it. Any less than that and it does slow to a crawl.
Superfetch only loads programs, not other data, and is a lot more sophisticated than simply loading the most recent things (that was what Windows XP's prefetcher did). Vista looks at both what programs you use most often and WHEN you are using them. Say you open a certain program every week day around 1:00. Superfetch takes note of this and loads it at that time. It dynamically adjusts based on both how much a program is used and what time of day/day of the week and any other pattern it can detect. And yes Win7 did improve on this further to make it lighter.
Third, when playing a game or any program that uses a large amount of RAM Superfetch drops whatever it is holding and gives all the RAM over to that application. This also happens if a large number of programs are open and together put a large demand on the RAM. Test this out for yourself. You can physically see how low it gets by comparing the RAM used before opening programs that use lots of memory to immediately after closing them.
Finally, the way you stated the DWM's memory usage was not clear, so I don't know if you know this or not. First all DirectX/OpenGL programs run directly on the GPU's dedicated RAM without any copy. Yes there are two copies of rastered program interfaces from programs that use GDI, but one copy is in the main RAM and the other in the GPU's dedicated memory. Win7 eliminates the copy in main RAM and exclusively uses the GPU's dedicated memory
Anyway, you seem to have a lot of misconceptions with what Windows is actually doing.
First, Windows XP Mode IS a MS Virtual PC. The bonuses of this over your own XP image are programs launch-able to your Win7 desktop (outside of the Virtual PC Window) without starting up the full Virtual PC and you don't need to provide a license. Now having your own image of XP does have its own benefits as XP Mode is a scaled back version of XP just to run programs and do basic stuff.
Second, Vista works perfectly fine on 2 GB so long as you don't have an large amount of programs open that use a lot of it. Any less than that and it does slow to a crawl.
Superfetch only loads programs, not other data, and is a lot more sophisticated than simply loading the most recent things (that was what Windows XP's prefetcher did). Vista looks at both what programs you use most often and WHEN you are using them. Say you open a certain program every week day around 1:00. Superfetch takes note of this and loads it at that time. It dynamically adjusts based on both how much a program is used and what time of day/day of the week and any other pattern it can detect. And yes Win7 did improve on this further to make it lighter.
Third, when playing a game or any program that uses a large amount of RAM Superfetch drops whatever it is holding and gives all the RAM over to that application. This also happens if a large number of programs are open and together put a large demand on the RAM. Test this out for yourself. You can physically see how low it gets by comparing the RAM used before opening programs that use lots of memory to immediately after closing them.
Finally, the way you stated the DWM's memory usage was not clear, so I don't know if you know this or not. First all DirectX/OpenGL programs run directly on the GPU's dedicated RAM without any copy. Yes there are two copies of rastered program interfaces from programs that use GDI, but one copy is in the main RAM and the other in the GPU's dedicated memory. Win7 eliminates the copy in main RAM and exclusively uses the GPU's dedicated memory