According to Microsoft
Here's What You Need to Use Windows XP Home Edition
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PC with 300 megahertz (MHz) or higher processor clock speed recommended; 233-MHz minimum required;* Intel Pentium/Celeron family, AMD K6/Athlon/Duron family, or compatible processor recommended
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128 megabytes (MB) of RAM or higher recommended (64 MB minimum supported; may limit performance and some features)
Therefore, those 486 PCs should not be able to run XP in normal conditions, it won't let you to install!
(But of course you can do it though someways, there are people using 68K CPU to run OS X, right?)
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