1: No you won't need a new computer for the new Flightsimulator but if you want the graphics maxed out you might.
2: No, it will NEVER EVER, EVER! be on an normal Xbox. Xbox only has a 800mhz CPU, most new games require over 2000mhz P4. Xbox is Direct X 8.1 compliant not 9 compliant so it CANT run Flightsimulator X.
FSX will be optimised for use with Windows Vista, so to get maximum pleasure from it, I would say either upgrade or yes buy a new one. I might be
No, that changed ever since Vista got delayed, infact, E3 is on a 7800GTX.