If it's been a while since your last PC upgrade, chances are good that you stand to benefit from a refresh to your hardware. While in theory anything with a Core 2 Duo / Athlon X2 (the later variants at least) or newer is still "fast enough", gains in efficiency not to mention system responsiveness are definitely worth pursuing. And if you're having any sort of stability problems on an older PC, trying to fix things can often end up costing just as much time and money as simply starting anew.
While picking out parts for a new PC is relatively simple on one level, the trick is in choosing parts that provide a good combination of price, performance, reliability/stability, and features. Despite advances in technology, the reality remains that going out and buying the cheapest motherboard often means you get exactly what you paid for: a cheap motherboard. That might mean things like compatibility issues with certain devices (e.g. DIMMs or PCIe cards), instability, and/or other issues. Troubleshooting such problems is no fun, so saving $20 at the cost of hours of lost productivity isn't something I'd recommend.
For other parts, however, the choices are pretty straightforward. CPUs/APUs rarely cause instability or other problems on their own, so the only real factor in selecting a processor is how much performance you want and how much you're willing to spend. I'll be listing several potential processor choices below. Storage and memory are for the most part compatible with any good motherboard and are largely interchangeable -- again, the decision mostly boils down to performance and capacity versus price.
