I'm thinking about making a computer with the shell of a ps3, but what are the dimensions of the inside of the ps3 casing? what size motherboard would be able to fit into the casing and would there be any room in it for a video card or would the motherboard need to have onboard video? Any help would be greatly appreciated.

I dont know the full dimension but you might be able to fit a ATX or a babe ATX.

Issue is clearing for the gfx unless you get an PCI-E adapter that alows you to laydown the gfx. Here a link of what you might use or need
Pci raiser (http://cgi.ebay.com/cheap-PCI-riser-card-32bits-extender-adapter-flex-cable-/320619481886?pt=LH_DefaultDomain_0&hash=item4aa6690b1e)

Once install you might not have any room for a PSU it will have to sit externally.

You can fit front side usb where the ps3 usb sits. and mod alittle more to fit front audio, Esata and firewire.

As for HDD you might have to use Psata *Portable sata* which are used in laptop. cause there 1inch thinner
Sorry this one for IDE 2.25 to 3.25 convertion (http://cgi.ebay.com/NEW-LAPTOP-2-5-3-5-IDE-HARD-DRIVE-DISK-HDD-ADAPTER-/380339929399?pt=LH_DefaultDomain_0&hash=item588e06c137)

As for the disc drive/CD/DVD/BLUERAY player. You can use or buy laptop disk drive and couple it with this
Laptop Disk drive adapter Sata (http://cgi.ebay.com/New-Laptop-Slim-CD-DVD-Drive-JAE-50-Pin-SATA-Adapter-/270659038849?pt=LH_DefaultDomain_0&hash=item3f04892e81)

:) If any luck you should be able to fit it all minus the powersupply into this case and have it running nicely with acouple 20mm/10mm fans :)

If you want to get more creative, drop in a water cool system. attach the radiator to the top and blend it all with fiberglass. autocar store sells a can full or you can buy the sheet and resine to make your costume fit

remeber think it out, make it simple, thats what modding is all about.