A desktop is designed to be opened, and that is its whole advantage: parts are standard, replaceable and resellable. It is also where people buy the wrong thing, because a part that is excellent in general can be physically impossible in your case.
Step 1 — Learn your four numbers
Before buying anything, write these down. They decide almost everything.
- Form factor. ATX, micro-ATX or mini-ITX board and case. Full-height cards and tall coolers do not fit small cases.
- Power supply. Wattage, age, and which connectors it has. A ten-year-old 500 W unit is not the same as a new one.
- Clearance. Maximum graphics card length (measure from the rear bracket to the nearest obstruction) and maximum CPU cooler height.
- Slots and bays. Free PCIe slots, M.2 slots, and whether the drive cages are occupied.
The case and motherboard model numbers will give you all of this on the manufacturer's site. Photograph the interior before you order anything.
Step 2 — The order that pays
1. Storage (SSD). If the machine still boots from a spinning disk, nothing else comes close. Boot from a minute to seconds, applications opening instantly, updates finishing while you watch.
2. Memory. If you run out, everything stutters. 16 GB is the comfortable floor; 32 GB for heavy multitasking or creative work. Two sticks for dual channel, and turn on XMP or EXPO in BIOS so you get the speed you paid for.
3. Graphics card. Only if you play games or do GPU-accelerated work — and only after checking length, connectors and wattage. It is the biggest single gain for gaming and almost nothing for office work.
4. Cooling. Dust, airflow and a cooler that is not marginal. Cheap, and it protects every other part you have bought.
5. Processor and platform. Board, processor, memory and often cooler together. Price the whole set; this is usually a rebuild rather than an upgrade.
6. Case and power supply. Worth doing when you are already changing something else, or when the existing supply is old, noisy or marginal.
Step 3 — Upgrade the platform, or replace it?
Processors are socket-bound. Most boards accept one or two generations beyond the one they shipped with, and the drop-in upgrades are often modest. Check the board's CPU support list before buying a faster processor for it.
Rough cadence for a desktop that is used daily:
- Years 1–4: storage and memory as needed, maybe a graphics card.
- Years 4–7: graphics card, power supply, cooling. The platform is still fine.
- Years 7+: the platform is the limit. A board, processor and memory set together is the next step — and at that point, compare the price against a complete new machine, including the case and supply you would not need to replace.
Replace rather than upgrade when the case is proprietary (many pre-built office machines), when the supply is non-standard, when the board is more than three sockets old, or when the upgrade total passes about a third of a new machine.
Step 4 — Reuse what you displace
Old desktops make excellent second machines — a home server, a NAS, a media box, a machine for the children. A five-year-old desktop with an SSD will stream, download and serve files perfectly well for another five years. That is the practical reason to keep the old parts when you upgrade: a spare power supply, a spare drive and a spare card turn a failed evening into a ten-minute swap.
What to skip
- "Gaming" memory, network cards and thermal paste. The labels cost money; the performance does not exist.
- Second graphics card. Multi-GPU is dead for gaming and rarely worth it elsewhere.
- Registry cleaners and optimiser suites. No measurable gain, real risk.
- Overclocking as a first step. It gives single-digit percentages and costs stability. Fix the bottleneck first, then consider it.
- Cheap power supplies. This is the one part where spending £30 more is genuinely the cheaper decision — a failed supply can take the rest of the machine with it.
The one-line summary
Check the four numbers, then buy in this order: SSD, memory, graphics card if you need one, cooling, and only then the platform. Everything on the list is standard and replaceable, which is exactly why a desktop is still the best value computer you can own.