INFO: this tweak is made for the mid range and low end cards that supports dx10 (ATI HD2x, HD3x, HD4x, Nvidia GF8x, GF9x) it could be used for high end cards but I don't think it's necessary
Yesterday I got Windows Vista Ultimate, I installed it with some fear that my games will be slower, but the big surprise is the contrary, the majority of my games runs faster now and with DX10 like Gears of War and Bioshock...
Far Cry 2 is one of the few games that uses the DX10 technology to bring great graphics
the bad thing about Far Cry 2 in DX10 mode is that the lowest settings available is "High"
well not anymore

first of all start the game and set the dx mode to 10, restart the game
when the game starts, go to options and you will notice that all settings are set to "high" or above and you can't go below high.
start or resume your game.
when the game loads, play a bit, if the game is slow and you don't want to lower the resolution, do this:
Go to options -> Graphics, set the dx mode to dx9, it will prompt you to restart, don't
what did you do now?
you fooled the game to use dx9 graphic options but the game is still in DX10 mode, so you can lower the graphics settings to a lower value than "high" but the game will still be in DX10 mode
when you want to exit the game, just go to options and reset the dx mode to dx10 and exit the game