i was out today and seen a mac at a car boot sale, it said 20 pound good working order. i know it will work as i have bought of them befor and there there every week. i think it was a g4 it looked like a ball and then the monitor was on top (it looked like a lamp) I thought it was a old one and would be rubbish but when i was looking they were selling for aroung 100 pound. if i get it could i connect to my wireless internet or what would i need. i have never had a mac befor so i dont know much and do you know how fast would they be. is it worth 20 pound.
Girugamesh and Yanni both have provided solid answers to your question, but essentially if you have modest expectations for the performance of a vintage Mac and don't mind the likelihood of additional time and expense to update it a bit, 20 quid is a small price to pay for a computer that remains quite usable for many tasks. It's also a really unique example of interesting industrial design for the right person.
Complete specs on the iMac line (including the G4) may be useful. Notably, the Geekbench benchmarks can give you an idea of the relative performance compared to a modern Mac:
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