Is it possible to upgrade a first-generation 2.66 Ghz 2-core Mac Pro (Woodcrest) to a faster model by replacing the CPUs? I've read that some Mac Pro models shipped with 3 Ghz 4-core Xeons (Clovertown).
Yes, you can but only with Intel Xeon E53xx, X53xx or L53xx series processors that have a LGA 771 socket.
Although one word of warning, the supported OS upgrade life time of the Mac Pro 1,1 and 2,1 looks like its coming to an end. Apple as we have seen in the news is prepping for Mountain Lion, and dropping support for all macs that lack a 64bit EFI. So it appears highly likely that Mac OS X 10.7 is the last OS you will be able to run on these Mac Pros, unfortunately. Be aware of that when prepping for your Mac Pro 1,1 and Mac Pro 2,1 upgrade plans, especially if keeping current is a priority of yours.
That said, now on to the upgrades:
Here are processor upgrades that people have completed and confirmed in Geekbench as working, see their results below.
Yep. Anand even did it when the Mac Pro first came out. I've seen other reports too.
Found out that a company does processor upgrades, but for some Mac pro models only. Perhaps yours is eligible:
And also this forum about upgrading a first gen Mac pro:
I've just done this upgrade to a used Mac Pro 1,1. I bought two X5465 CPU on eBay from China (production Costa Rica) and they installed fine with heat around 40 degrees at idle and 50 degrees under full benchmark testing with Open CL Sala room test (CPU). I did use very good heat paste though after thoroughly cleaning the processors.
I want to run Snow Leopard so I'm not bothered about the 64 bit EFI. But if I was, the computer when I bought it was running Yosemite using Piker's bootloader (just puts in some translation files in boot.efi which thunk the 64 bit calls onto the 32 bit firmware). No problem running even high end graphics like Borderlands 2 on Steam on a Nvidia GTX 580. If you plan to game, though, I don't recommend the Mac Pro 1,1 as it has only a PCI Express 1 (maximum bandwidth 2.5 GB/sec) which hobbles high end cards (higher than Radeon 5770 or GTX 285).
I'm not going to follow my own advice and I'm dropping in a Radeon 5870 as there is an improvement especially for FCPX and apparently the 5870 is quieter. Price difference these days is more like $50 and not $200 between the 5770 and 5870.
Even the Nvidia 7300 GT runs fine and absolutely silent. It does choke a bit when trying full screen video playback with both 30 inch and 24 inch monitors attached.
As work computers (as opposed to Da Vinci Resolve 12 colour correction suites or high end gaming rigs), the Mac Pro 1,1 is an amazing silent and powerful performer. I suspect these elegant silver towers will still be in service when people tire of black trash cans without slots for cards or drives.