Tip: Get a broken G5 and fix it. It's usually cheaper.
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In general, I'd say that's horrible advice unless you REALLY know what you are doing.
G5s are superb when they work right, but can be nerve-racking when wrong.
I'll buy broken G4 towers all day, as I can almost always fix them or at least isolate the problem.
Once you move beyond the basics like upgrading hard drives, RAM, expansion cards and optical drives G5s are a nightmare to work on. Parts are hard to access, even harder to diagnose, and tough to find once you do.
I WILL NOT buy a G5 unless it will at least chime. I've bought G4s that were not much more than a shell. That's how much I think of working on them.
To answer the original question, though-
The first generation Mac Pros have their issues like difficult to find(and expensive) RAM, but in general are solid machines. The biggest advantage they have in today's world is software support. Although officially they aren't supported by the current version of OS X, there are plenty of guides to help you install.
You'll find that prices on the secondary market track pretty closely with the amount of RAM installed. If you are going to go the Mac Pro route, I'd suggest sourcing one with at least 4gb of RAM and preferably more. OWC sells the correct RAM for these, but it's fairly pricey and it's better to just get it already installed unless you get a really great deal on one.