langlo94: It is possible to do it that way. Look up the color code for the molex connector. The yellow wire is your 12v+ the black wire next to it is 12- or ground. The next black wire is 5v- then the red wire 5v+. When I mounted a USB HDD w/switch internally in my computer I cannibalized a molex connector from a SATA to molex adapter. I removed all four pins from the housing. Then uncrimped the 12v pins and inserted my power wires. Then recrimped the pins. Never had a problem with that setup. If they decide to go 5v with the RasPi you would just do this with the red and it's neighboring black wire instead.
Find a pci card that is bolted to the expansion cover. Does Not matter if the card works or not. Remove the expansion cover. Build an adapter plate to bolt to it. Then bolt the RasPi to that. Use a dremel to cut the needed holes through the expansion slot cover for the video connectors. You will have to build a couple of USB adapters if you are going to connect a mouse and keyboard to the RasPi.
Doing it this way you will have to establish your network through a hub, switch or router. My suggestion of connecting through a network card is a bit harder but keeps everything inside the case. either method will work. I would also try to support the tail end of the RasPi by cutting a piece off of that old PCI card, scratch the traces off of it or cover them. Then attach it to the RasPi and slip it into the PCI connector. This will reduce the chance of vibration weakening and snapping off the RasPi from the slot cover.