I was looking at a very cheap Dell PowerEdge DCSCS24-SC and it has 2 Intel Xeon L5420 CPU's in it. Since I very highly doubt it scales perfectly, how would this computer compare to a modern day quad core of similar stats, a single L5420, and a Core 2 Duo 3.33 ghz.
It's not really so much about speed as it is smoothness. The real benefit to having multiple CPUs is a dual set of inputs and outputs. If you take a quad core CPU, and you have a situation where two cores finish their task at exactly the same time, then only one core can offload its data and the other one is forced to wait. Same as if two cores both need more data at the same time. Only one can pull new data at a time.
So what you end up with is a system that is much more responsive under load, it's not necessarily faster. A single 1GHz CPU would probably be faster than 2x500MHz CPUs, but 2x500MHz CPUs would result in a system that is considerably more responsive even under load.