dual cores are great. you can get far more than double performance in some instances. take the example already mentioned: gaming and encoding. if you try this with even a top of the line single core you will get rock bottom FPS (tested with a 6800gt, FX-57 and MS Flight Simulator, i think resulted in 5FPS ) and poor encoding performance. with a dual core you will get almost full performance from both applications although a bit lower due to memory bandwidth constraints. in video encoding you will get effectively double the performance. with the price of dual cores steadily dropping i don't see any reason not to get one as the prices are far lower than buying 2 of the equivelant single cores.....thats value .
the pentium D's aren't all bad it depends on what you are using them for and your budget.
I'm pretty confident that encoding items and the flight simulator do not support dual-core CPU's, that is, unless you were talking about doing both simultaneously, in which, a dual-core processor would surely pull ahead. As already stated, dual-core is only useful when you are doing multiple things at one time that require a lot of "juice", or using an application or game that supports dual-core processors (basically, none, hopefully more items will support it soon though). So if you are constrained money wise, a single core will suffice. However, Intel has already dropped single core CPU's altogether in there newest line of CPU's, after the Pentium series is dropped (rather soon I'd imagine, before 2008 probably), all their CPU's will be dual and quad core. So in all honesty, a dual-core CPU is probably the way to go, as it's actually hard to find a solid single core CPU that is not overly priced (FX series for example ... )