I may have misread your post, but you cannot put a 667 or 800 or 867 or 1ghz Titanium DVI logic board into a 400mhz case because it is a vga case. The screens are also not interchangable. Hard drives on a Tibook are very easy to change out.
Your best bet if you like the Tibook style is to get a 500mhz board and have Daystar upgrade it to 550mhz. Or overclock it yourself if your 400mhz board is still alive. The 550mhz board alone also does not fit into a 400 case for some reason.
Believe me I did my homework on old Tibooks.
I owned the 400, two 500s, one 667, and a 1 ghz in my time.
Though this would not preclude you from making a hackintosh out of spare parts if you are really good with a soldering gun, and a whizz at electronics and case modifications.
The old 400 screen was lower resolution than the later PBs, too.