What game are you playing? If the game is optimized poorly then that would explain your problem. For example, StarCraft II only uses 1.5 cores. So my CPU will never reach 100%. Since I have a high-end video card it will only use 75% of its overall power. This would result in what looks like a problem with your graphics card, but in reality your CPU is bottle-necking the game because the 1.5 cores that it is using are maxed out. Overall CPU usage is much lower though when looking at all 4 cores.
EDIT: Any game that has a ton of players, or units, generally rely more on your processor than your graphics card. SC2, World of Warcraft, any kind of MMORPG. Most of the time your GPU is waiting on your CPU for commands. If the game is designed poorly and only uses 2 cores, it appears that only 50% of your CPU and 75% of your GPU are doing something when they could be doing more. blah blah blah.... try overclocking or disabling hyper-threading to improve single thread performance.