a new logicboard is just not worth buying , a refurbished one maybe , or a used one thats still working ,but they are rare to find and expensive too , you are better off selling all internals and external parts that are working separate and get a new one , which cost you only a couple quid more really on top what the new logicboard would cost , or a used iMac but working will actually be cheaper then the new logicboard (but the risk that it will break soon too)
i was lucky the last owner of my iMac core duo(2006) had under apple care replaced the logicboard and display and harddrive and the powersupply too in 2009 just before apple care ended , so with luck (touch wood) it should now last to 2012
but a shame you have to pray that these things happen while under apple care if you want to use a Mac longer then 3 years
