Look through the forums, there is a lot of opinion out there.
A few key points.
It must be 3,1 of later, 1,1 and 2,1 do not do Mavericks without some help and from what I've read are not worth the effort.
You can use non-Mac graphics cards, I have a GTX660, so long as you don't mind not having a boot screen, but also be careful about power, some cards are power hogs and would require additional power to the Mac Pro.
Be careful about RAM. A 3,1 uses DDR2, and some think that it is expensive, but the price has come down, so it is about the same as DDR3 now, just not as easy to find, but that's what the internet is for.
The nice thing about the old Mac Pro is that it takes pci-e cards, so it is expandable, but not every card will work, so you need to be careful. You can also swap out the processors, but you need to make sure that what you buy will fit, there are differences in them that means that they are not usable in each model.
Look at the benchmarking sites to see what type of performance you can expect. While Mac Pros are generally fast machines, they have their limits and you should be aware before you drop your money and go down a route that limits your build.
I bought 3,1 in 2008 and have upgraded it with a second quad processor, a new graphics card, more RAM, USB, lots of internal and external memory, and I am thoroughly happy with the result, knowing that I will get a few more years of use out of it.