We are running Nehalem Quads now and the drop in CPU "load" has been amazing. I wish I could get the six core in a single CPU version but have only seen them in "monster" servers designed for VPS with dual six core CPU's and huge drive arrays to handle lots of VPS accounts.

Before the end of the year 8 core CPU's are supposed to be coming allowing a quad CPU server to have 32 cores--unbelievable.
With the performance increase we have seen with the Nehalem quads I can't even imagine those, even with dozens of VPS clients. The resources available to each VPS will be greatly increased or the host can choose to put more VPS's on every server.
AMD is supposed to have something along the lines of 8 cores "up their sleeve" and they claim to have prototype 32 core chips using a new technology beyond silicon to allow the hige increase in the same space/power "footprint".

These days with price points where they are I can't see using less than some type Quads unless you were doing a dedicated for just one site hat needed the disk space but not the CPU power.

Now with DDR3 RAM the disks are the bottleneck and SAS drives are still expensive, As they are coming down I plan for our next expansion to be dual CPU (whatever Nehalem is available that suits our requirements) with SAS disks to increase the ability of the disk to keep up with the CPU power. Current SAS technology gets the best performance at 73 or 146 GB --too small for all the CPU/RAM power available so we're waiting for bigger/faster SAS drives to become more efficient and less expensive then the processors--wel who knows what they will have then?