The 6870 is way better, and the 2100 won't bottleneck it, so it's definitely the better combo. The GPU is more important for gaming, as long as it's not limited by the processor.
Why would one be better now but worse in 2 years?
it depends on what resolution you are gaming at, ie at low resolutions the CPU does more work but at higher res. the GPU does more. The i3 2100 is powerful enough though to not hold back even a high end card at low res. You don't really need the 2500K unless you are running 2 x 6850 or better, which you don't need unless you are running very high res. anyway.