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Technically you need the "big heatsinks", but I've been using 8 - 2gb sticks with the "thin" heatsinks in my 1,1 for almost 2 years now. I believe the 4gb sticks run hotter so I wouldn't recommend the thin heatsinks on this but the 2gb ones have been fine for me.
 
I have two of the thin heat dissipator FBDIMMs in my MP2,1 and in general they run about 50F hotter than the Apple-approved equivalents.
 
And have you had any problems?

None, but it does heat up the other DIMMs on the board and there is a rise in ambient temps as well. I've only had them in for about 4 months. I was careful to get server memory (although I think that all FBDIMMs were server RAM). My Apple heat spreader RAM runs at about 120F normally and the thin spreader DIMMs run at around 175F. I would hesitate to recommend them just because they're so much hotter. They're cheaper in price, but the corresponding rise in heat scares me.
 
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