Image editing in Adobe Photoshop graphics editor is definitely not one of those types of tasks where multi-core CPUs come in handy. Many operations performed by the editor in fact use single-threaded algorithms. That is why clock frequency becomes of greater important in this application than the number of computational cores.
During data archiving six-core AMD CPUs outpace quad-core one although it works at higher frequency. However, they are still very far behind Core i7-920. It must the relatively slow memory subsystem of AMD platforms that determines this outcome. When desktop analogues to the server Istanbul appear, we may expect the performance to increase a little, because these upcoming CPUs will be compatible with Socket AM3 and hence will support faster DDR3 SDRAM.
Distributing computer client called Folding@Home can use the potential of multi-core processors very efficiently that is why Opteron 2435 looks pretty good here. It proves just as fast as Core i7-920.