Just add me to the latest list of MacPro1,1 owners who have upgraded to 2 x Xeon 5355s. The upgrades to the MacPro1,1 this past month: 1. Upgraded from 2 x Xeon 5150s (Dual core 2.66 Ghz) to 2 x Xeon 5355s (Quad core 2.66 Ghz) 2. Added 16GB RAM - Total installed 24GB 3. Upgraded firmware to MacPro2,1 firmware 4. Upgraded to Mavericks using SFOTT (Sixty Four On Thirty Two - Uses Tiamo's boot.efi). Now the MacPro's performance is competitive with modern Macs. Not bad for an 8 year old computer! I'm hoping to get another 4 years of use out of it now.