It looks like you've already made the decision, but if you're going to get one for free, even the first generation, I would recommend taking it -- I use a similarly big machine (with similarly big power-sipping capabilities) minus a video card, and it's great. I've got sixteen gigs of ram in my machine and I run a bunch of virtual machines with it -- it's great since most of my individual "machines" don't necessarily need a whole lot of CPU horsepower on their own.

If it's a 2008 model, that's all the better because you'll have better CPU options, and it looks like the 2008 models support up to 32 gigs of ram, if you want to put a bit of money into it.

I've also heard that VMware ESXi works on the Mac Pro hardware, and the free version of vsphere5/ESXi5 allows for up to 32 gigs of ram. You could take the video card out and run it as a remote virtualization box, which can be handy to have.

And if all else fails, once you've kicked the tires, you can sell it wherever.