Im a bit confused here. You say you own a GeForce FX6700 ... that does not exist so I guess you mean a FX5700. And thats the reason the game can only run in medium settings. The game is simply so demanding when it comes to graphics power, that your FX5700 only comes in as a medium card.
This (again) proves that the power of the graphic card is more important than that of the cpu. I know you cannot run FarCry with a small cpu like a 1ghz (not in high settings anyway)... but a 2 or 2,5 ghz would most likely be able to run high/very high settings if it was fitted with a strong card like a Radeon 9800PRO/XT. The reason is high/very high settings mostly bring graphic card side details that only affect vertex and pixel shader performance (like more shadows, more detailed water etc) while the number of raw polygons only raise a bit and textures generally not being much more detailed compared to medium. High/Very high also applies more filtering (anisotrophic) of textures and antialiasing which is again work only affecting the graphic card.