I think you get bigger bang for the buck spending your dollars on render farm machines. Four (cheap) machines is substantially cheaper than a quad-proc/quad-core single box. I just put together a farm of sub-$500 nodes and find it was money well spent.

It also isn't fair to claim that a dual core or dual proc system is the same as two nodes on a farm, since all the cores/procs on a single box have to work with the same memory modules (the farm nodes do have to pay the network overhead to get started, but in a complex render, that's a trivial percentage of the time). It gets worse if you're just looking at Hyperthreaded procs. While a HT proc will keep a single-threaded app like Poser from bogging the machine down, it won't approach anything like 2x speed increases on renders (in fact, in a time trial between my wife's PC and mine, I found the Vue render time of the two machine was only minutes apart out of hours...even though my CPU is a HT P4 processor running at a touch faster than her P4).

Finally, I think the most underrated value a render farm has is that it keeps your main computer free for other things. For example, I work in both Poser and Vue, so when I send my job out to the farm, I often contintue to do test renders and work in Poser, which is something I couldn't reasonably do if I were letting Vue do the render on my main box.