Most of those who know the real benefits of an AMD or an Intel processor, at the processor level, usually build their own boxes based on their preferred CPU.
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This recent announcement probably will help AMD more than Dell and in fact, is already helping AMD. AMD doesn't really advertise much. But now, with a very top tier name like Dell making huge headlines about AMD, suddenly AMD is becomming more of a household (and corporate) name.
Some of the effects will be rather immediate and will increase over time, particularly when those same blind corporations start hearing about AMD processors in their own DP Departments. "Buying daughter a PC for graduation... hearing good things at work about AMD... let's try a PC with one of those."
Its only the underinformed that would claim corporations are "blind" just because they aren't buying cpu's from your preferred cpu vendor; corporations have vastly different requirements and desires besides only the best cpu for the job. Any company still buying Xeon servers from Dell, most likely does so for good reason (single source supplier, support, good mix for them of ras, features, price,..), not because they are blind or clueless, even though the cpu's or system architecture might be second class compared to opty..
To prove they are not blind, in just a few years opteron conquered 40% of the conservative 4S market, without Dell's support that is. Opteron 8xx really doesn't need TV adds or full paper adds from Dell, and Dell embracing it will do precious little to its credibilty in this market. Its already more than credible there: in fact, it is nor only credible, but even so wanted, corporations are switching vendors just to be able to buy Opterons. Dell could no longer ignore its customers crying out for Opterons, so it decided to offer them. Don't tell me this will increase AMD's perception. People/corporations buying 4+S systems are rarely underinformed or brainwashed by tv adds.
Dell embracing and agressively promoting A64 or Turion for consumers might still help though, even though AMD's "street credibility" is already an order of magnitude better than what it used to be. But in mid range server market, AMD just doesn't need an introduction anymore, certainly not by Dell (it will still help sales though, especially to companies single sourcing from Dell).