http://www.microsoft.com/games/flight/

Looks pretty neat. Requires Games for Windows Live.

MINIMUM REQS:

CPU: Dual Core 2.0 GHz
GPU: 256 MB card capable of shader 3.0 (DX 9.0c compliant)
HD: 10 GB hard drive space
OS : WinXP SP3 or newer
RAM: 2.0GB