2 & 3. most of the component processors are at least 65watt tdp. there is this celeron 35watt one though.. i use a 65w pentium dual core with a dstax..
other then hijacks 6-7 zone system, i have not seen a true need for a graphics card. onboard is usually plenty even for playing back HD movies.. if it is really a concern, look for a board with something better then 'intel' graphics..(my zotac has nvidia geforce 9300 integrated)
4. in order of power consumption--most to least: 3.5"-->--2.5"-->--ssd. i would always prefer to use either 2.5's, or ssd's in car mostly because of their small size and lower power consumption
6. if you decide to use win7, it would be best to use at least 1.5gb of ram. i found that any less tends to slow the pc down.. but if you are using hibernate, then 1gb is better for faster hibernate/resume times..
7. some have run m2/m3's with i3 boards, but i am just not comfortable recommending them-- i would say to get at least a dstax, or m4. if not a opus 360...