Getting onto the Intel Upgrade path with a Z68 chipset and GEN3 motherboard seems the best build option currently.
It performs great now, and it should continue to come out on top following future upgrades.
Id also seriously consider saving your money for a little while longer if you are unable to get the i5-2500k. The i5-2400 just doesn't have the same potential. Over-clocking and SandyBridge go hand-in-hand and co-exist together to achieve some awesome 4.5GHz+ clock speeds