Do not buy, imo. This is the LOWEST MODEL Mac Pro 1,1 and in order to be even remotely usable would require significant upgrades. I wouldn't pay more than $200 for a 1,1 - especially one without an upgraded GPU and the lowest grade processors available in that machine. Reasonable CPUs (E5355s, which are Quad Core 2.66GHz) run about $50 for an upgrade, and RAM can be expensive, though I just picked up 16GB for about $45, then you would need a new GPU to take advantage of the Tiamo hack for Mavericks.
And even after all that, you're still running 2006 hardware with a GPU running in a PCIe 1.0 interface with 667mHz RAM. The upgrades and a reasonable GPU are going to cost $100+ depending on what you wanted to do, to upgrade, but I can't imagine anyone using that machine as a daily driver in its current hardware state.