If I were to make a system, I'd probably keep it for three years at the very least, minus perhaps a CPU upgrade. I think the 2500K is a good CPU for now and a year from now, but its multi-threaded performance isn't THAT great. The 2600K nets you a ~20% improvement there, but I don't really know about paying $310 for a Quad-Core. I'd really prefer to get a Six-Core CPU, but the 3930K will be a whopping $585 while platform costs are higher than P67, too. At the same time, the upgrade path of P67/Sandy Bridge isn't that great.
Ivy Bridge will net us a ~5% improvement in IPC and probably 300MHz or so higher clock speeds at the same voltage (1.35V), but nothing earth-shattering. Going from a 2500K to a theoretical Quad-Core with HT Ivy Bridge isn't that big of a deal, except perhaps for the PCIe 3.0 support (as long as the motherboard supports it, of course). At the same time, there's some rumors flying around that Ivy Bridge-E will eventually support Eight-Core consumer CPUs, and if that happens, they'll be the new $1,000 ones and a new, slightly lower clocked model will come out and cost less (see Core i7-980X to 970). Then again, I think paying $1K for a CPU is ridiculous and I wouldn't do it.