One of the most important factors when buying a computer is expandability. It seems really cool to buy a really tiny computer that fits in any place but it will stink when you want to upgrade it 2 years later. When you buy a computer look for:
1. The amount of pci slots for adding more peripheral cards
2. Whether it has a pci express graphics slot for upgrading later(essential for gamers)
3. How many ram slots it has and how much it expands to
4. Is it big enough to add more hard drives(some micro computers cant be upgraded at all)