no.
1. You're correct in assessing that the 7 series of nvidia cards won't have DX10 because the technology simply isn't there much like the X1000 series won't either.
2. You'll only benefit from a larger MB on the video card if you increase the resolution or use a game that can take advantage of the extra memory. The aperture value in the bios from what I recall just sets the allocation of video memory in the system memory through the AGP bus. This was the main feature of AGP but it never really took off because the system memory is far slower than the graphics memory.
what he said