First unless you're looking at an Ivy Bridge forget DDR3-2400, you might add +0~4FPS from DRAM Frequency alone with DDR3-2400 vs DDR3-1600 'IF' you OC the CPU (best case), B-U-T you add a much higher risk of instability as the 'cost.' You're many times better off getting a better non-reference GPU and OC'ing the GPU with MSI Afterburner which works on any recent and current GPU.
/edit - 8GB (2x4GB) is plenty enough for gaming, and if you want 16GB you'll have less issues with a 4x4GB kit DDR3-1600 CAS 8 or 9 memory. 8GB/stick especially at higher Frequencies or lower CAS Timings also adds unwarranted instability; 8GB non-ECC kits still need some work and improved IC's. BF3 example: 3.0GB~3.5GB game & maps + 3.0GB~3.5GB Shared Memory (GPU) + 1GB~1.5GB OS & background.
Bottom-line, spend the money on other stuff -- namely a better GPU.
1600 is definitely the sweet spot and great value for money, try and get the lowest CAS timings you can for example 8-8-8-24 will perform 0.5% (guessing) better than 9-9-9-27. Hey it's nothing, I know, but it's good to know