It is a silent and low consumption HTPC for gaming and movies possible?

See the form in my sig... Doesn't sound like the best idea to me, but it depends on what games at what settings and what exactly you mean by silent.

I know that some sort of compromise would be necessary... I'd like to have a powerful HTPC to use it with my Bravia 46'', next to my PS3 and Xbox 360, the problem is that a HTPC able to play 1080p Flight Simulator X needs an i7 + good videocard (so good cooling is mandatory). Other sims I'd like to play are Life For Speed (no problem) and other racing sims like rFactor, etc.

Are you considering water cooling your system? That would keep the performance parts cool and minimize the noise.