Ben,
I also think the tall posted cape is a great design, whether you're building 2.5 stories or 1.5 stories.
I really like the queen post design that Daiku suggests. I used a modified version of the bent in his picture for my own house. What I did was to push the queen posts outward, and call everything between the queen posts "living space" and everything beyond the queen posts "closet space." The queen posts delineate my knee walls. (You can never have enough closet space.) My queen posts are approximately 6 feet tall and they intersect the rafters instead of the collar tie. The elevation of the bottom of my collar tie is about 6'8". This is pretty low for ceilings, but normal for door height. Aesthetically, the perceived height of the room is much higher, since the ceilings are vaulted all the way to the peak of the rafters... I'm just saying that you need to make sure the collar tie is not a head bumper, but don't get too hung up if it is lower than where a ceiling would normally be... unless you use the collar tie for ceiling support and turn everything above it into a proper vented attic (which ain't a bad idea either).
My main wall posts extend about 18" to 24" above the continuous 24' wall tie (the wall tie is also part of the floor system for the top floor). If you make posts extend too far above the wall tie, then as has been said, the posts will be straining to keep the rafters from spreading. On the other hand, if you make the posts extend too little, the rafter-to-post intersection is close to the post-to-wall-tie intersection and the post is weakend by the joinery there. Traditional joinery has the rafters intersecting the wall ties directly... a much stronger solution, but this would not be a "high-posted-cape," and without extending those outside posts upward a little bit, it would be hard to get much living space from beneath the rafters.
Don't forget, like Daiku said, unless it is pretty far down on the rafters (at your knees or waist in a high posted cape!), the collar tie is in compression (keeping the rafters from sagging, i.e. pushing the rafters apart), not tension. (Very non-intuitive, but that's what I've read and experimentally confirmed in software). So, necessary as it may be, when placed in the upper half of the rafters, the collar tie is exacerbating the rafter-feet thrust issue at the tops of the posts, so queen posts are a _really_ good idea. Long live the queen(s).
-Thomas
BTW, that's an absolutely beautiful frame Daiku.
