hay relay depends on what you want from the computer, but for 50 american you cant go wrong. (i live in the uk mac pro 1.1 cost like £200+)
you will get a nice stable computer which can hold 4HD's with no problem and relatively quiet for it's size. (and an extra 2 if you relay want)
you can buy HD's and a GPU and at a later point get a newer macpro and swap over your drives if you want.
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(can only swap ram in to a 1.1/2.1/3.1 and not a 4.1+)
what do you want to do on the computer ? thats the main thing it will be ace for holding lots of files, productivity apps and some gaming (maybe not max settings on the new games but still ok with a newer GPU)
photo editing will be fine, light video editing etc..
add up the price of ram and HD then compare to a mac min, the cheapest one will be faster and quieter & cheeper to upgrade
but slightly less cool
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and will not hold 4 internal drives if thats what you want.
also might be a fun project
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what computer are you on now? are you expecting it to be an upgrade ?