Bowsing around the listings for the Athlon 64 FX-55, i've notice alot of things that unsuspecting bidders may fall for.  Here are some tips to help you be the informed buyer about this top of the line chip.

The Athlon 64 FX-55 processor comes in 2 flavors, the Clawhammer and the San Diego chipset.
The difference?

The Clawhammer uses a 130 nm process die type, which means it uses more electricity(runs a a little bit hotter, also uses voltages at 1.5v) and is less efficient than the 90nm San Diego process type (1.35-1.45 voltage..  Smaller process type=more efficient and uses less electricity at the expense of being more expensive.  Last time i checked, the difference was $10 more for the San Diego.
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The San Diego core uses these multimedia instructional sets: MMX, SSE, SSE2, SSE3, 3DNOW! Professional.

Clawhammer:  has the above multimedia instructional sets except SSE3.
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And thats the difference between the two.  Basically, the Clawhammer core came out first, then the San Diego.


Other things that i want to talk about is the warranty.  If you buy the retail (not OEM) package, you get a Manufacturer's warranty for 3 years parts/labor.  OEM warranties, depends on whose selling it.

Your best bet is to  go with the Retail Processor because of the warranty, and you get the heatsink/fan. 

OEM, no heatsink/fan and warranty.

And quicknote, please do your research, either at:
www.nextag.com    /has a past item price history, to see how much the price has fallen/risen
www.pricegrabber.com    /see who is giving the best deals

Usually you can find the market price for the processor, and use that as a rule of thumb when browsing ebay.