yes
RAM, storage, Motherboard, PSU.
Check the power cable and the PSU.
The motherboard... Interconnects - CPU - HDD - RAM - Optical Drives - GPU - PSU
Motherboard RAM Hard Drive Disk (HDD) CD/DVD Rom PSU CPU Network card (if not build into motherboard) Graphics Card
Yes, you can. Just make sure that you have a right kind.
The PSU, Power Supply Unit
The older format, AT, will power up without a motherboard. ATX form factor needs motherboard connection to power up. However, for about $15 you can purchase a PSU tester that plugs onto the psu to allow it to power up without the motherboard. Try: dalco.com, or FRY's etc. Actually, ATX will start without a motherboard, or a 'PSU tester'. The ATX motherboard power connector will have 1 green wire, and several wires of other colours. Find a bit of lose wire, it doesnt need to be very long. Make sure both ends of the wire are stripped. Push one end of the lose wire into the 'hole' that the green wire connects to, then push the other end into any 'hole' that has a black wire leading to it . The PSU should start, and stop when you remove the lose wire. == ==
Bad PSU, (power supply). Did not install motherboard right in case, and bad ground.
Grey wire is used to report to the motherboard that the PSU has passed all tests and ready to work. Some motherboards have hard times recognizing good signal, as result the motherboard will not start.
Proccessor (CPU), video card, PSU, Motherboard, Hard drive, Memory/RAM. the Network card and the sound card are normally integrated on the motherboard, but these are required too.
If you're at least in the 300W range, you can run a lower-spec GPU fine. Generally speaking as long as your computer motherboard supports PCI Express, they are backwards-compatible (but you may not see the benefits of buying a top-of-the-line GPU for a really old computer). If it does not support PCI Express, you will have a difficult time, but not impossible, to find some old VGA or PCI graphics cards on the cheap somewhere.