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it'd really depend on my needs. do i already have a spare monitor, keyboard and mouse laying around? if so, i'd get a mac mini. do i need portability? maybe a used powerbook or possibly even a macbook might be right. it depends on your individual needs. if i had only $600 to blow on a mac, i'd try to fetch a used 1.83 ghz macbook. it might take a bit of time to find, but that's the model i'd go for.
 
For me I need portability first so I'd probably look through craigslist for either a 12 or 15 inch powerbook g4. They're immensely capable laptops and I miss my 12 incher very much.

If I needed a desktop I'd probably pick up a 17 inch imac g4 and if I had a screen, mouse, and keyboard I'd get a refurb intel mac mini.
 
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