As most people have already explained here (though I did not read the responses thoroughly), when you buy OEM you're basically getting a stock board with reference drivers. Nothing that is up to date.
Depending on the circumstances, this may mean your video card might not perform as well as a fancy vendored card, and may be more likely to break down.
OEM is the opposite of brand name.
It's like saying, well you buy a car. An OEM car could come from a used car salesman who just does a shoddy job and wants to sell you something. Now if you buy it from someone who cares, they may give it a new paint job, make sure the tires have enough air, put a CD player in it for you, etc....
In some cases there is little difference as when looking at the base of things, you are still getting the original and same chipset (no matter what vendor it comes from, or if it came directly from ATI's production factory). In other cases it makes a HUGE amount of difference.
IMO it really depends on the part you are buying. For things like CD-ROMs and keyboards, OEM is ok. For important parts that effect performance/stability/compatibility OEM is a big no no (important parts would be like your motherboard or video card).
With a brand video card you might get a much better cooling fan and heat sink, the latest drivers, bundled software, tweaking tools (like overcloking software), and possibly additional card features like video capture (though most ATI cards come with video capture originally).
I would not recommend OEM for a video card (or motherboard). You could see a significant frame rate difference. Though I feel it is ok for things like floppy drives that don't effect your system very much.
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Benchmarks don't lie 🙂