I am fairly new to Mac as I bought my Mac Pro early 2008 so I don't know much about the older ones before that so I have a question. Does your 1st Generation 2006 have an Intel processor? If not Snow Leopard won't work with it...
The current aluminum tower case makes it easy to mix up the Mac Pros and Power Mac G5 models. The Mac Pros from the outside look very much the same except for two ODD tray slots in the cheese grater front.
Internally they were very different. G5s came with three generations sporting IBM 64-Bit!!! PPC970, PPC970FX and PPC970MC processors. The FX was a die shrink of the 970 to 90nm and the MC was a dual core of the same architekture as the FX. Introduction was 2003, 2004 and 2005.
The switch to Intel was 2006 with the first Mac Pro using 2 dual core Woodcrest Xeon 51xx CPUs in the LGA771 socket. From todays perspective it is also relevant that this MacPro1,1 model had only a 32bit EFI. All Mac Pros had quad configuration compared to the G5 which had Quad only in the top model of the very last 2005 series. Internally RAM was positioned on two riser cards and the PSU went from the bottom of the case to the top. The two SATA ports of the G5 went to 4 HDD and 2 ODD ports which was a massive improvement. The logic board is based on an Intel 5000 design with 6321ESB ICH chipset.
2007 Apple made a silent upgrade and added a 3,0 BTO Octad configuration in the same socket with Clovertown X53xx Xeons. The X51xx and X53xx are fully compatible and the machine is designed for up to 150 W TDP. Everything else stayed the same.
2008 Apple kept the 771 socket and logic board but used the Intel die shrink (tock step) from 60 to 45 nm. Again there are dual core and quad core CPUs. They are the X52xx and X5xx series. Due to the die shrink the firmware needed adjustments and Apple went from a 32-bit EFI to 64-bit. The logic board remained very simillar but got PCIe 2.0 with faster slots. The 2006, 2007 and 2008 models are all competitively priced and retain value very well on the used market.
2009 Intel was on the tick step and introduced the Nehalem micro architecture. Apple was the first major manufacturer who got quantities and went to market. Nehalem is a significant change of practically everything but the case. The socket went to LGA1366, the logic board is based on the Intel 5520 design and features a ICH10R chipset. There are no IDE channels any more and so the ODD went to one of the previously unused ODD SATA ports. Instead of RAM risers both CPUs with heat sink and RAM are positioned on a daughter board. This time Intel dropped the dual core approach and offered only quad CPUs. They designated them the X35xx single socket and X55xx dual socket Xeons. Apple acommodated that change by designing different daughter boards with one socket and two sockets. The CPUs are not interchangeable. Nehalem introduced a new way of memory management which was integrated in the die. Nehalem CPU have no front side bus FSB any more but use the new QPI quick processor interconnect technology for memory communication. The heat sinks are designed for 130 W TDP officially. They have internal fans for the first time. Xeon X55xx CPUs are fitted without heatspreaders which makes CPU upgrades a bit of a pain. The different memory architecture brings different RAM fitting strategies for optimum use of the memory channels as well. Nehalem is back to hyperthreading compared to the previous Core micro architecture. It means that each core can be split for two threads simulating two virtual cores. So a quad can run 8 threaded programs and an octad 16-threaded. Apple made a huge price hike with Nehalem and upped comparable models by 800-1500$ according to MacRumors experts. This price hike and the obstacles to easy CPU upgrades made the Nehalem Mac Pros less popular then previous model steps. If the improved bandwidth justifies the price hike is somewhat diputed. In my view it benefits the power top end users only and should have not hit the mid and low range models.
