"After unsuccessfully searching the web for reports of Early 2008 Mac Pro CPU upgrades, (I suspect due to cost/benefit - with quad-core CPU(s) already, the cost of faster CPUs in the past probably made that less attractive. Over time prices fall, more used CPUs become available, etc.) I finally just did it and wanted to share with your readers the ease of upgrading the Early 2008 Mac Pro.
My Mac Pro was originally a single Quad Core 2.8Ghz model.
The 2.8 CPUs are reported to be Intel E5462 CPUs. I was able to find reports that the mother board was the same used in the dual 3.0 Quad core and those CPUs were E5472's.
I found a matched pair of E5472's on eBay for a steal ($202 shipped - used ones on eBay) and ordered them. I was worried because the 2.8GHz CPUs are listed as SLANT and the 3.0's are listed as SLANR. This didn't mean anything to me, and since the price was low enough I went ahead and ordered them.
I also ordered the Mac Pro replacement heat sink from eBay (lowest price I could find) which ended up costing me $215, but did include thermal paste and alcohol pads, which are nice if you don't generally have that laying around. Basically i searched for the mac part number (076-1303) which lists as being for the 2.8 or 3.0Ghz CPUs.
The first stumbling block was finding a 6" (or longer) 3mm allen wrench. Which I found at a local hardware store (I live in Japan, so its unlikely that a reference would help you).
(some previous 2006 Mac Pro upgrade reports mentioned this tool and sources. For example an Elkind 3mm hex (9-10" long) "T" wrench from ACE Hardware (USA) for appx $4. Local tool/hardware stores (even Sears) may also sell similar tools. One reader from Germany bought one from a (BMW) repair tool source after googling for it.)
After that, you basically pull the heatsink cover off the CPUs (magnets), pull out the 1st and 2nd hard drives (or caddies if no drives), unscrew the two screws holding in the fan tray (one at the tope under where the drive caddies are, and one right at the bottom by the lower heat sink, its at a weird angle). I pulled the video card out too, just to have more room.
Then I pulled the upper heat sink blank out, and the real lower heat sink (this is where you need the 3mm allen wrench). The lower heat sink has a plate on it directing air. I put both freshly cleaned E5472's into the CPU slots, put thermal grease on both of them, replaced the lower Heat sink and added the new heat sink to the upper CPU. The new heat sinks come with a plate similar to the one on the lower heat sink in the event you are replacing that heat sink due to damage. if you are using it on the upper CPU, the plate is unnecessary.
Of note.... before you re-assemble. Make double sure you re-connect the original heat sink to the motherboard and connect the new one. If you don't, your Mac won't get a temp reading and will run the fans at full speed. (I read this many times myself and forgot the top one anyway.) It's just a small black plug with 2 pins.
Overall it was as easy as can be and the Mac immediately reported 8 3.0Ghz cores. Now I can dedicate more cores to transcoding (streaming media) or for my Virtual Boxes.
(he later wrote)
When I initially put it back together, I forgot one of the thermal sensor plugs on the heatsink. The fans ran at full speed when I turned it on, but it still worked flawlessly. I pulled it back apart and connected the plug and since then it is as silent as possible. Barely audible even up close.
I would give this upgrade (overall cost/benefit) a rating of 9. Easy. Very effective, but I wish the heatsink cost less. If the heatsink can be found at better prices, the value would be better. If I already had 8 cores, an upgrade would have been less beneficial. With the early 2008 4 core model though, this makes more sense.