I buy almost everything on-line. It is much cheaper. In the case of OEM vs. Retail, if your buying with intention to overclock and dont have to physically manipulate the chip then go with retail and the 3 year warrant. If your just buying to clock at 800 then I would check out oem w/ a 1 year warranty. Most of the time a processor is either bad or not. They don't often go bad on their own. Usually a thermal problem causes them to go bad. Just as a rule of thumb, I usually by retail for myself. The cost is not that much more, and you can usually buy a retail processor online for much cheaper than you can at a local retail store.

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System #1
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AMD T-bird 850
Epox 8KTA+
128MB PC-100 Micron Ram
Maxtor 20.4 GB 7200RPM HD
44X CD-Rom
Lucent 56K Modem
Turtle Beach Montego II
Intel 10/100 Nic
19" AOC Monitor
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System #2
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Intel PII 450
Aopen AX6BC
Maxtor 13.0 GB HD
Maxtor 4.5 GB HD
Memorex CD-RW
Acer 40X CD-Rom
128MB PC-100 SDRAM
ATI 8mb All-In-Wonder Pro
2 Voodoo2 8MB in SLI Mode
Yamaha Sound Card
Intel 10/100 Nic
17" Monitor