If you take 'Crysis' for example that game was made so much ahead ,at that point in time regarding hardware, so it still give some setups trouble running even at a highend level,but at the same time it was still backward friendly enough so you could get a good game experience even on a fairly low-level PC.
Now that said SH5 (of course pure guessing here) will probably be made backwards-compatible friendly, but still with enough horsepower in the graphic-engine-department, that if somebody want to "choke" their PC, "be my guest" so to speak.
No softwarehouse/publisher want to completly cut any part of hardware/software market out of the equation that is still in "mainstream-mode".
So I think that DX9 will be the start level and then the sky is the limit....
The Gb? well...I think it will be 1GB minimum and 2GB recommended,,and if you have more,the better of course....