The other responses to this have been woefully speculative at best, plain wrong in others. The short answer is to rent one for a month and benchmark your particular application.

I was surprised to discover that with a workload composed entirely of XML parsing using Python+lxml, an OVH 1.8ghz Atom N2800 beat hands down my mid-2010 2.4ghz Macbook Pro. This isn't a crazy result either. For instance, while the Macbook is equipped with dual cores, each core supports only one hardware thread, and thus is blocked while waiting to complete a load from RAM (a common occurrence when dealing with giant cache-unfriendly DOM trees that don't benefit from the Core 2's much increased L2).

In contrast the N2800 has 2 cores with 2 hardware threads each (hyperthreading), so each core can progress while its buddy thread is stalled. In my particular workload this was enough to beat by a significant margin a much faster, expensive and power-hungry processor.