The 2006 Mac Pro could only officially do OS 10.7.5
and OS 10.10 is going to be out soon.
A tech in CA worked on these Macs for awhile and found that Apple had locked them up so they would not run an OS beyond Lion. But he created a code to unlock them so they can run Mavericks, Yosemite, and El Capitan. Currently, you can purchase hard drives on eBay with modified OS's already installed and configured. They're just plug and play, but due to the demands of these later OS's, you must change out the stock graphics cards for either a PC or Mac card with more RAM. I have run these Mac Pro 1,1's with an Nvidia Quadro card with only 512mb RAM just fine, but one might want more. An ASUS GT610 Silent card with 2GB RAM works great, but you don't see the Apple logo during boot. However, PC cards can be flashed to show the logo during boot. Mac cards will cost more, but do show the boot time logo, if this is important to you. RAM is cheap: about $20 for a 16GB kit of PC-5300 @ DDR2. The replacement graphics cards however run at DDR3. I buy the 1,1's and do the mentioned upgrades and then resell them, primarily to photographers on a budget, and all are happy. Not sure about recording though, but mine come with Apple Logic Pro X. I have read that the 1,1 can render batches of photos faster than some of the later models.
Another possibility is to install Linux onto the 1,1. As I understand it, Linux is the ideal environment for audio recording as the clock speed (latency?) is faster than those for either PC or Mac. Add to this that all of the recording software is free. One can get/download a free copy of Ubuntu Studio, which is the Linux OS plus all of the included recording software absolutely free. I have installed Linux Mint on the 1,1 in the past and it runs faster than the native Apple OS on the same machine. I then added hand picked recording software to the installation. The DAW software of choice seems to be Ardour, used professionally. And lastly, the most recent issue of Ubuntu Studio is supported until 2013 (Long Term Support version). One can actually have two hard drives installed, one booting El Capitan and the other booting Ubuntu Studio.
https://forums.macrumors.com/threads/best-gpu-choice-for-upgrading-a-1-1-macpro.1962225/
Add an SSD or cheaper SSHD, and you can double the stock speed, maybe.
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The 2006 Mac Pro could only officially do OS 10.7.5
and OS 10.10 is going to be out soon.
A tech in CA worked on these Macs for awhile and found that Apple had locked them up so they would not run an OS beyond Lion. But he created a code to unlock them so they can run Mavericks, Yosemite, and El Capitan. Currently, you can purchase hard drives on eBay with modified OS's already installed and configured. They're just plug and play, but due to the demands of these later OS's, you must change out the stock graphics cards for either a PC or Mac card with more RAM. I have run these Mac Pro 1,1's with an Nvidia Quadro card with only 512mb RAM just fine, but one might want more. An ASUS GT610 Silent card with 2GB RAM works great, but you don't see the Apple logo during boot. However, PC cards can be flashed to show the logo during boot. Mac cards will cost more, but do show the boot time logo, if this is important to you. RAM is cheap: about $20 for a 16GB kit of PC-5300 @ DDR2. The replacement graphics cards however run at DDR3. I buy the 1,1's and do the mentioned upgrades and then resell them, primarily to photographers on a budget, and all are happy. Not sure about recording though, but mine come with Apple Logic Pro X. I have read that the 1,1 can render batches of photos faster than some of the later models.
Another possibility is to install Linux onto the 1,1. As I understand it, Linux is the ideal environment for audio recording as the clock speed (latency?) is faster than those for either PC or Mac. Add to this that all of the recording software is free. One can get/download a free copy of Ubuntu Studio, which is the Linux OS plus all of the included recording software absolutely free. I have installed Linux Mint on the 1,1 in the past and it runs faster than the native Apple OS on the same machine. I then added hand picked recording software to the installation. The DAW software of choice seems to be Ardour, used professionally. And lastly, the most recent issue of Ubuntu Studio is supported until 2013 (Long Term Support version). One can actually have two hard drives installed, one booting El Capitan and the other booting Ubuntu Studio.
https://forums.macrumors.com/threads/best-gpu-choice-for-upgrading-a-1-1-macpro.1962225/
Add an SSD or cheaper SSHD, and you can double the stock speed, maybe.