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I had a K6/2 in my computer about 5 years ago. It was the cornerstone of the PC enthusiast scene for a long time, due to the performance being up there with the pentiums, for around half the price.
The Socket 7 motherboards this chip ran on were the first to embrace new design philosophies of ATX and various user friendly improvements like colour coded connectors, jumperless setup, BIOS screens that didn't require a manual to understand. Basically the coming of age of PC technology, removing many of the barriers for people that wanted to be able to upgrade their own systems.
A system running a K6/2 would still be perfectly usable today. I wouldn't reccomend anything under the 500mhz mark, and don't expect to be able to play any modern games with it. But for Games from before 2001, Web, email, Word, etc etc it's still fine.
Very stable chip and although they can run quite hot, you don't need a massive amount of cooling to get it stable.
I wouldn't expect to pay any more than a fiver these days, as you can get a chip a hell of a lot faster than this one for £30. But if you do come across a K6/2 you could do worse than put it to use.