Upgrade first gen mac pro in 2019..... worth the effort and what parts?

You should check out the various reports at: http://www.xlr8yourmac

There should be plenty of information there that would help you get something later than Lion installed on your Pro, as well as tips on what upgrades can be added.

I have a 1,1 Mac Pro that I use as a Wayback Machine (that'll probably only make sense if you watched Rocky and Bullwinkle). For those purposes, I haven't had any reason to do the EFI hack. It runs Tiger through Lion. Snow Leopard runs great on it. I don't attempt to use it for Internet access, just to run older software. I wanted to add a USB 3 card to it, but ultimately had to abandon that idea if I wanted to keep Tiger on one of the drives. I had a first-gen CalDigit card worked great with Leopard through Lion, but caused a kernel panic if I tried to boot Tiger.

which Mac Pro is it by year or processor speed?

mac Pro 1,1 2006 or 2,1 2007 are stuck at 10.7.5 unless you use hobbyist Hacks to extend them.

Mac Pro 3,1 2008 and 4,1 2009 will run 10.11 El Capitan directly, higher with hobbyist hacks.

You won't be able to use a recent version of OSX on it so getting online or getting updated apps will be difficult or impossible. Memory for Mac Pro's is usually more expensive than for other systems as most of them used ECC RAM. You will want to use only RAM from Crucial or OWC as Macs are very particular about the memory they use. 2GB of RAM is not very much and will likely need to have more installed to be useful.

Since you won't be able to use a recent or supported version of OSX on this computer, you might want to try using one of the Ubuntu Linux flavors or Linux Mint both of which should install easily and be friendly to new users. Both of these operating systems are free and can operate in low memory situations if you choose the correct Desktop Environment (LXDE, XFCE, MATE). In my experience 4GB of RAM even with Linux is the minimum RAM necessary to have a decent online experience, but you easily test it yourself. You will need to learn a new OS and expect to make mistakes. This also allows you to run tests on the system to make sure there are no hidden surprises before spending money on upgrades.

If I were you, I would make sure to zero out the hard drive before using it. You don't want to have anything from the previous owner come back to haunt you.

That is the Dual Core original 1,1 version from 2006.

There is a hobbyist hack that can allow the installation of the 2,1 firmware, which (I think) would allow you to upgrade the processors to Quad core X5365, if you wanted.

The biggest problem is that it has only 32-bit firmware. Hobbyists discovered you could ADD the 64-bit firmware (on top of, and dependent on the built-in 32-bit firmware) by making patches to the EFI and substituting a slightly modified boot loader. Doing this would make it "look like" a 2008 model, and would allow you to run up to 10.11 El Capitan.

We are generally discouraged from posting direct links to these unsupported and unauthorized modifications, so that the innocent and unqualified do not hurt themselves by attempting hobbyist modifications way beyond their abilities. But we can provide some suggestions and additional guidance, if you just ask.

You must have 6GB or more of Real RAM to get 10.11 El Capitan or later to run without constant Paging. The DIMMs for these Mac Pro models have the enormous heatsinks, and they are required. If you can arrange your RAM DIMMs so that all slots on a card are fully populated, the big "trick" of these FBDIMMs is that they switch to double-width memory accesses (i.e., faster).

If you get through all that, you can add a SSD drive on a sled with a bit of fiddling, and pick up faster Applications launches and better speed overall.

Whatever you end up doing, it was a cool thing to find. It’s like something from a time warp, in great shape, and running the original OS. If you succeed in getting it up and running with El Cap I’d love to see a follow up report.

The 5150s means it is a 2006 (MacPro1.1)

The main reason that machines slow is because it does not have enough RAM, not because of deficiencies in the older software it can run.