Hi there, thank you for your nice feedback. Have you looked in my signature cause I have made updates in the past few days.
1) size of ITX is not a constraint, I only want one GPU and never want to CF/SLI. Ever. Also I only want an SSD and maybe in 1-2 years I'll add a storage HDD. I currently have 40GB filled on my 2-years old laptop even though I put everything I have on it, so 128 will be good for me, trust me
2) Overheating is no problem the case has front intake/rear outtake, great for ventilation and the i3-2120 does not produce a lot of heat much less when the Ivy Bridge i3 comes out and uses even less watts.
3) The overall system will max out around 250w due to the low consumption of the i3 and the low consumption of AMD cards in general. Actually the PSU is 2x more than the system will load at, so yeah more wattage is just going to be a waste.
The graphics card (6870) is much more robust than the 550 Ti which you linked
4) The power supply I picked *is* modular