- Power supply: min 400W, 500w should be sufficient for most purposes.
- Mother Board: supports your preferred CPU, requisite no. of PCI / PCIX / PCIExp / AGP slots depending on what you have to fit in; built-in SATA, sound (not video), network, etc.
- Processor: dual core, speed - most you can afford
- RAM: min 1, pref. 2 GB, whatever type and speed mobo requires
- Video card: best you can afford.
- HDD: SATA, biggest you can afford
Then compare the cost of building that against buying new from whoever.
Upgrades: Best components you can get that are compatible with existing setup. If you start changing mobo, then you are really building from scratch.
SD