The heatsinks aren't hugely important. I have Amicroe server RAM in my Mac Pro 3,1 - which is 667 mHz, (Mac Pro 1,1 RAM) and though it does have a relatively large heatsink compared to normal DDR2 RAM, they are nowhere near as sizable as the 800mHz "Apple Approved" modules that came with the machine. Not sure what the appropriate price is for these modules, my friend bought a server about the same time as I got my MP, and it turned out it didn't need FB-DIMMs, got these for free. It took some finagling to get all the modules recognized, but I think that has to do with the two different memory speeds as opposed to the brand identifier. However, I can safely say I have all 14GB up and running. If you can find FB-DIMMs for anything less than the average price of "absolutely absurd" I would go for it.