OK to "build" a PC without a case?

The primary function of the case is cooling. That is, it DIRECTS airflow where needed (if it's well designed) to keep certain components cooler.

Some people think that opening up the side of the case keeps a computer system cooler, but this only keeps the ambient air inside the case cooler. A case with a side open can lead to hot spots...because the airflow is disrupted. That is, the air may be cooler, but it's not going where it needs to, to keep components cool.

I'd suggest that before you get the case, you should have a box fan or something blowing air over all the components, to compensate for the fact that you are going to have a lot less airflow where it's needed.

Also, the case has the power switch. You'll have to very VERY carefully short the two pins on the motherboard marked POWER_SW (momentarily) with a screwdriver or something to turn the computer on.

The main problem I can think of with this plan is expansion cards. The case usually supports the expansion cards (like video cards in particular). I'm not sure how you can secure the video card in a way that will handle the weight/stress of the monitor cable. If the video card works loose while powered ummmmm....that's NOT good.

A better plan might be to get the absolute cheapest case you can find, even if it's a real piece of garbage. Build in that, then get your favorite case later?

I think that you will need a piece of plexy plastic 3/8 thick to set the motherboard on and secure the screws. Build it upsidedown like ( Meaning, that the power supply is at the bottom in the end.) even though when it goes in the case all you do is have the processor in the regular position. Man, I would wait and just buy the case you want.

There is a major risk of electrical shock and damage to the parts due to spills, sneezing, static, etc

There is also risk of short circuits from metalic/conductive items touching the motherboard.

Apart from that, if the units are put together carefully (there will be no support for any PCI/AGP cards inserted) and put out of reach of water/tampering... it should be fine.

I have a custom-built PC that I built into a wooden cupboard running quite happily and it has been for years (its my homes fileserver and also doubles as webserver and email server, administered via remote login as it has no keyboard/mouse/monitor)

One of the more extreme mods that Im hoping to try in the future is full submersion of the motherboard, etc in brake fluid or distilled water - as its non-conductive (and could be built with another container infront that contains fish in normal water)... but this isnt for the faint-hearted or modern machines you dont mind blowing-up if your wrong :oP

Always remember: Safety first!

you can set it up, test it, whateve... just be careful not to fry it. I find the best way to do this is to avoid static (rub your hands on the old case before tweaking), and use non-metallic surfaces to work on. cardboard boxes work well (did mine on a pizza box - whatchout for xtra cheese!)

GL