I used to operate on a two-year upgrade path: every other year I'd build a new machine, and in the years between, I'd make some individual upgrades (additional RAM, additional disks, faster GPU). Now I'm looking more at a 5-year path, with individual upgrades every year or two between. I really think that, at this point, anyone with a machine built in the last 3 years has little to benefit from a total overhaul. There are a few areas where everyone is likely to see real improvements:
- Operating system: if you don't have Windows 7, get it, along with any upgrades required to meet the minimum specs. I can't recommend Windows 8 for any user for any purpose at the current time.
- RAM: if you have a 32-bit system (unlikely if it's less than 3 years old), you should have 4GB of RAM. If you have a 64-bit system, you should have 8GB; possibly 16GB for computer audio, video, or graphics professionals, or users running intensive virtual machines.
- SSD: you should have an SSD. Honestly. If you don't have one, get one. They're getting cheaper by the day, and will give you a real, noticeable performance improvement across the board. Your SSD should host your OS and applications, at the least. Let Windows 7 handle optimizing system configuration for the SSD; just do a clean install onto the SSD and let it do the rest. Ignore all the "SSD tuning tips" that require changing OS settings or disabling services. 99% of them are wrong, and the other 1% are debatable.
- Display: IPS displays are a world away from your typical bargain LCD, and they're getting cheaper constantly. You can now get a name-brand, 24" IPS display for under $300. If you're a computer professional, you probably want at least two.
This is, of course, a generalization, and depending on how you use a PC, you will have different needs. Audio professionals will obviously see benefits from discreet audio hardware. Imaging and video professionals will want a fast CPU. 3D graphics professionals will want a fast CPU with as many cores as they can get, as well as a fast GPU. 3D gamers will want the fastest GPU they can get - possibly upgrading GPU every year to 18 months.
My system is two years old now. So far I've upgraded from 4GB of RAM to 8GB, and I've added a 256GB SSD drive. I'm planning on replacing my single 23" TN LCD with dual 24" IPS LCDs soon. In another year or two I'll probably upgrade the GPU, and in three years or so I might be in the market for a total replacement - or I might not. I wouldn't be shocked to find that, three years from now, brand-new hardware doesn't put enough distance between itself and what I've already got to make it worth the money. Only time will tell.