<b>O</b>riginal <b>E</b>quipment <b>M</b>anufacturer.
In short, cheap and cheerful. OEM parts usually fill pre-built systems and come with no frills. Depending on the hardware part you may want to go for the retail version.
e.g.
CPUs - Retail version usually comes with HSF.
Drives & Cards - Retail versions come with Manuals, cables, bonus software.
I do know that the retail version of the 64MB Radeon had higher refresh rates guaranteed.
Retail guarantees may be longer.
<b><font color=blue>~ Whew! Finished...Now all I need is a Cyrix badge ~ </font color=blue>
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