Man, this is the third time today I've noticed one of my posts just disappeared...
Anyhow, the two main differences I've noticed are consistency (as mentioned before), you're more likely to get a bad stick if it's generic RAM than brand name. The other is overclockability. My generic RAM won't run even 2 Mhz above spec, but most name brand RAM will.
Reliability while running is another issue, but I've found that's a lesser difference than the other two. Typically problems will be from a stick that was DOA and you didn't even notice for a while (it happens).
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My Athlon can beat your Ferrari off the line.