The PC Gamer $653 Warhead Machine
Crytek built its own machine capable of running the game "silky smooth", with "all the bells and whistles turned on". Considering that it has an 8600GT, I'll take it with a grain of salt, but Dan Stapleton confirmed in a PC Gamer Podcast thread that they absolutely did play the game on this machine, and it was running on High details.
Shuttle SG31G2S Barebone Case - $239.99
Intel Pentium E2180 2.0GHz Dual-Core CPU - $82.99
Samsung SH-S203B DVD Burner - $29.99
Hitachi Deskstar 7K160 160GB hard drive - $48.99
A-Data 2GB DDR2 SDRAM - $45.99
MSI GeForce 8600GT 512MB video card - $94.99
Vista Home Premium - $109.99
Don't forget that you can easily find a case/power supply solution for cheaper than that! It would compensate for the mouse, keyboard, speakers, and monitor you might need.