If you are not refreshing datacenter, you are (probably) losing money.
According to a 2007 survey, in 3 years data centers spend more on power/cooling than the equipment it supports! In reality, probably 4 years is more like it.
Recent CPUs (like sandy bridge) have brought down their heat signature as well as power requirements while nearly doubling the performance. Add virtualisation and you can halve the average power requirement again (if not the peak time load). So basically the investment pays off for itself in a couple of years.
Plus, it will save on real estate cost if the load goes up beyond current data center's physical size. One of the problems is that a bunch of netbursts are still running stuff and no one wants to touch it. Probably 10 of them can be replaced by a 1U Xeon today (just guessing).