I've been wanting to jump into the Mac scene for awhile now, and these new iMacs are exactly the excuse I've been needing to force myself to do it. An iMac with a dual-core Intel chip? Man, two years ago that would have been unthinkable.
If you're motherboard was "bad", I think you'd probably have some other issues as well. I hate those kinds of answers, and I would probably take it somewhere else on principle - even if I needed a new motherboard.