Do not buy generic off the shelf FB-DIMM memory for a Mac Pro (I'm assuming it's a 2007 or 2008 Mac Pro). If you do, odds are it won't have the proper heatsink, and the Mac Pro fans will go into full speed mode.

I'd recommend the people over at OWC for Mac Pro memory.

Also, I'm glad I have ECC memory. Seems one of the modules I bought off a friend when he was selling his Mac Pro went bad. OS X simply kept running fine, instead of introducing random stability problems due to 1 bit errors. Yeah, some RAM might be cheep, but I'll pay a little bit more to get a friendly "Hey, memory is going bad" error instead of having to spend time tracking down problems. Not going to pay insane OEM memory prices though.