Just a report on my successful upgrade from a Mac Pro 2006 2.66 quad (5150s) to a 3.00 quad (5160s). It was pretty easy, but I can't stress enough having the right tools, particularly the long, thin 3mm allen wrench. You need about 10" of length to get down to the heatsink nuts on the motherboard. I thought my adjustable screwdriver with extender would work, but the extender was too fat for the cutouts in the heatsink. You need a rod no more than 5/16" wide. Also, though people have said that a 15 Torx works (it does), I actually think the 3mm hex is the perfect fit. It may be tough finding these locally, even at a home depot, so you may want to buy online ahead of time unless you want to splurge and buy a set of long allen wrench keys (~$25 and up). The other challenge was connecting the lower heatsink's temperature sensor cable to the motherboard. If you have fat fingers like me, an angled hemostat or needlenose pliers will help you get into the tight space. And lastly, a small vacuum cleaner would be useful to suck all of the dust bunnies off the heatsinks and the fans and out of the motherboard's nooks and crannies. While you're in there, you might as well clean it out! My computer is a lot faster, about 12-14% faster, with no noticeable increase in fan noise. I really wanted to get a pair of X5365 quads, but they are too damn expensive. This processor upgrade along with my SSD boot drive should hold me until the next generation of Mac Pros appears next year. Special thanks to forum posters bozz2006, Tesselator, JPamplin, and gugucom for their helpful tips in other posts. Now, if anybody with a 2006 Mac Pro 2.0 ghz quad is looking for a pair of 5150s, please PM me before I put them on ebay in a day or two.