I know this is an older thread...but...FWIW
Normally it is far less expensive in the long run to buy a new system rather than replace a proprietary motherboard. It really does not matter what name brand OEM computer it is, eMachine, Gateway, HP, Compaq, or Dell as all use a custom design motherboard for their systems.
Custom design motherboards provide the OEM retailer exclusive proprietary rights to their product line, combined with a substantial discount for volume pricing and licensing. Both Dell and HP/Compaq utilize Foxconn, Intel, or Asus as motherboard providers, but the motherboards are NOT off the shelf retail consumer designs in any case.
Sometimes you may find one of these OEM motherboards available from a supplier that purchases overstock from the aforementioned manufacturers. Then, and only then, will you have the option to buy an OEM design motherboard outside of the brand name storefront. Other retailers such as the Geeks will strip down running (return lease) systems to sell off the hard to find parts, then scrap the rest of the machine. So the bottom line is the parts, although scarce, are available if your timing is right. Now the guarantee they are good parts may be dubious to say the least.
Just surfing through eBay I can buy a used working Acer Aspire, or eMachine desktop computer with all the manuals, original CD's, and parts for less than one third the original cost, and all less than a year old. Often these low end systems will come with the original monitor as well. $250 US for a complete system including printer, monitor, speakers and the lot is but a fraction of the original price, and only slightly more than the cost of a new motherboard from the same manufacturer. A running econobox (just the CPU tower unit) like eMachine or Acer can be purchased for peanuts, often less than the cost of a new motherboard from anybody.