You also want to consider any whitebox opteron system being on the compatibility list of your storage vendor, as well as RedHat, etc. With EMC you can file an RPQ via your sales contacts to get it approved, though not sure how lengthy/painful that process might be, or if it's gonna be worth it.
Read the article devoted to the v40z on anandtech.com.
I am also trying to get a quad-Opteron versus the latest quad-XEON from Dell (6850), but it's hard to justify a difference between a 15K dell versus a 30k v40z for a 5-8% performance gain (read the XEON Vs. Opteron Database comparo on anandtech.com)...
15k vs 30k is indeed a big difference. But also realize that Postgres has a specific benefit to Opterons versus Xeons. The context switching storm happens less on an Opteron for some reason.
I would venture a much greater benefit than 5-8%, more like 10-50%.
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