If you want to see FPS improvements, you'll have to OC your rig. Generally, the lowest accepted speed amongst the enthusiasts is 3.0GHz. But to address the actual concern, using a matched pair of RAM to enable Dual Channel will offer slight gains, but not much.
Dual/Triple Channel architecture was designed with the intent to reduce the bottleneck that happens when the FSB of your CPU is greater than the speed of memory. In short, more channels equal more data transfer.
Something to consider...
Rules to Enable Dual Channel Mode
To achieve Dual Channel mode, the following conditions must be met:
Matched DIMM configuration in each channel
Same Density (128MB, 256MB, 512MB, etc.)
Matched in both Channel A and Channel B memory channels
OR
Populate symmetrical memory slots (Slot 0 or Slot 1)
Configurations that do not match the above conditions will revert to Single Channel mode.
The following conditions do not need to be met:
Same brand
Same timing specifications
Same DDR speed
Memory channel speed is determined by the slowest DIMM module populated in the system.
*This really depends on your motherboard. Some manufacturers use a color
-coded system (meaning use the same colors) and some require that you use the symmetrical approach (meaning same slot on both channels).