This week I bought two new peripherials for my computer: an USB 2.0 controller card and a sound card. Before you ask, yes, neither of those are integrated on the motherboard. Originally it wasn't a desktop machine but a server.
The first problem I encountered was when I tried to insert the A-Link U2P4 USB 2.0 controller in a PCI slot. It just didn't fit. The PCI connector on the controller had different keying from the PCI slot on my motherboard. While I could have made it fit using a hacksaw or something, that didn't sound like a good idea. I didn't understand the situation at all, so I looked up the PCI page from Wikipedia. What I learned was, that there are several versions of the PCI bus. The card I had in my hands was built to use 5 volt signalling and my motherboard used 3.3 volt signalling.
Further looking into different PCI versions revealed that the last version of PCI standard allowing for 5 volt only cards was deprecated already in 2004. It seems, that all new PCI cards should be able to use both 5 and 3.3 volt signalling or even 3.3 volts only.
The second problem was with the sound card, a Terratec Aureon 5.1 PCI (aka. Fun). I was able to plug that one in, but that's just about how far I got. Whenever I tried to turn the computer on, it would turn the power on for a split second and then power off. Just enough to make the fans twitch a bit. I don't really have a way to tell for sure what's the reason, but it seems that the card was short-circuiting some of the power lines, and thus causing the power supply to cut the power.
It may have been, that I just had an faulty card at my hands and if I exchanged it for a new one, it would have worked just fine. Also it may have been that the card is faulty by design and assumes that the signal voltage and +5 volt supplies are at the same voltage, thus shorting the +5V and signal voltage pins. What makes me to even suspect the latter is that I found notions that similar things have happened and that the sound card package showed a picture of a 5 volt only card. If the actual electronics hadn't been revised when the board was changed to universal type (both 5V and 3.3V) that's pretty much what would have happened. I already returned that card to the store for refund, so I don't have real evidence to back up that claim, so as I said, it could have just been one faulty card among a bunch of working cards.