There are definitely situations where storage isn't a bottleneck. In those applications, spending a lot of money on an SSD doesn't make sense. That's usually the case for netbooks and nettops, which are so grossly underpowered anyway that solid-state technology would really only be useful for improving ruggedness or making a dent in power use. SSDs also won't help when you're running workloads that specifically tax CPU and graphics resources; when either of those two subsystems are a bottleneck, no increase in storage throughput is going to help push more data through them.
However, our test results demonstrate that it does make sense to install a fast solid-state drive, even in a several-year-old system. It'll speed up the boot process, accelerate application loading times, and enable snappier responsiveness. In some cases, it still makes more sense to spring for a processor of graphics upgrade first. But we also saw examples in PCMark 7's suite where an SSD makes more difference than any other component.
SSDs aren't typically in consideration as an upgrade for older machines. Processors, graphics cards, and memory are certainly more common choices. But if you're spending most of your time in office productivity apps, browsing the Internet, or watching video/listening to music, an older Core 2 setup is still respectable. And anything you buy to modernize it is going to require some replacing. A new CPU necessitates a motherboard (and probably memory) as well. A high-end graphics card could stick you with a power supply upgrade. But so long as your platform has a SATA controller with AHCI support, adding an SSD is about as painless as it gets.
So long as you're running at least an Athlon 64 X2 or Core processor and at least a couple gigabytes of memory, an SSD could end up being the most significant upgrade available to you. Of course, if you're a gamer, a new graphics card is almost always going to be the way to go. For everyone else, though, an SSD will almost certainly facilitate more performance than a RAM upgrade, and it might very well out-class a new processor, too. You don't even need to go all-out on an SSD. Simply adding a 64 or 128 GB drive for your operating system and most performance-sensitive apps can make all of the difference. You're looking at a game-changing piece of hardware for somewhere between $120 and $250 bucks.
With all of that said, none of the machines we built were enthusiast-oriented setups. The conclusions we're drawing relate to the mainstream folks trying to get more life from an already-aging box. If all-around performance is your raison d'être, we have to acknowledge our enthusiast roots and recommend ditching that four- or five-year-old technology and discover the wonders of a PC built from the ground-up with balance in mind.