1. Why is it so expensive
2. Can i buy this kind of memory (mac pros memory) from a pc resseller or only apple?
3. Is it true that DDR2-SDRAM (PC2-5300) 667MHz ECC fully-buffered DIMM (FB-DIMM) are discontinued?
Question 1: like previous types of memory, the faster it is the more expensive it gets. Of course, I haven't purchased DDR2 memory yet as most of my other computers still use DDR1.
As for number 2, you can get it anywhere. AFAIK, it's just standard DDR2 memory. Nothing too special about it. You should be able to find comparable RAM from places like NewEgg, but I would check places like RamSeeker and Crucial's website for configurations.