What do you do when a Macintosh motherboard ceases to function, you don't have a warranty, and you do not have money for a Macintosh motherboard? Well, you use a PC motherboard. WHAT?! WAIT!? HUH!?
It really isn't that difficult. The ASUS P5KPL-CM has a great track record, as do many of the Intel Desktop boards. Main things to keep in mind are that Intel processors (Core Duo, Core 2, and i5/i7) are easier to work with than AMD, and Nvidia graphics are easier to work with than ATI. Also, Leopard has more support for various hardware than does Snow Leopard. So how is it done (new install recommended)?
Well, first you will want to cruise on over to Kexts.com and search for kernel extensions (drivers) that will support your hardware. You need to be very, very specific. Download those, and put them somewhere that will be accessible. The next thing you need to do is download Empire EFI, and again you need to be specific to your CPU. You burn Empire EFI to a CDR, and leave it in the tray. Reboot. You will come to a boot screen, and at that point you swap the disc for your OSX installation disc (best to use the retail copy that sells for $30.00 US). Hit f5, and once you see your disc on the boot selection, go ahead and hit enter. You will launch into the installer. If something goes wrong, you can try again. This time press tab once you have highlighted the install disc. For boot options use "-x -v" and note what the boot hangs on. A quick google of the errors will usually yield some information on how to solve the error. A classic one is "still waiting for root device". In your BIOS you need to see if you can switch your HDD mode to AHCI, and you need to make sure that the filesystem on your HDD is one that is supported by OSX. Usually, this means it would need to be either HFS(+), FAT, or NTFS. Some of the Linux disc formats make the installer freak out.
After you've completed the installation, swap out the OSX install disc for the Empire EFI disc. Boot with selecting your HDD. You will come to a desktop that probably looks pretty bad (low resolution), and you most likely won't have a lot of device support. First, double click the Empire EFI disc icon, and find your way to the myHack installer. This is the easiest way to make your system bootable. During that install process be sure to look out for the "customize" button and see if any of your hardware is mentioned.
After this is taken care of get Kext Helper, and install the kexts you downloaded earlier.
Quite a few people seem to have problems with graphics. First thing, do not install the voodoo graphics enabler for SL until you've done a software update to 10.6.4. Intel HD graphics chipsets are often supported after upgrading to 10.6.4 as well. If you have mouse lag, this is usually solvable by installing the AppleUserUpstream disabler. The frame buffer disabler is also convenient if you have the PC version of an Apple card.
Audio is sometimes problematic. Often, the voodoohda kext will work. When it doesn't options are slim. You can find kexts for most common audio chipsets, but when you can't it's time to buy an external audio card that supports OSX. For wireless cards, most broadcom chipsets can be made to work, but otherwise I recommend an external adapter there as well. Hawking makes some excellent ones.
If nothing you try works, I highly recommend looking into some of the OSX86 distributions that litter torrent sites. iPC and iDeneb are both very common, have a ton of kexts, and can be used on Intel or AMD. The only thing I advise here is using Leopard not Snow Leopard. I would also recommend that you have a legal copy of OSX of the same version. The Apple EULA openly states that OSX is to be used on Apple Branded Hardware only.