Crucial Memory Advisor
Crucial references every system we could think of, and offers referenced RAM directly through its online store for most systems. It was very thorough, and we were able to find even obscure systems from major manufacturers. A few modules no longer available through Crucial's store were listed as such, and it's worth noting that Crucial has never been a RDRAM producer. Crucial waives its $3 base 3-5 day shipping fee for any order $39.99 and up.
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Checking the Dell Dimension 4600 recommendations, a small notice on the left side of the screen, which reads "Important information about your system: Installed in pairs of modules" should be more prominent. Crucial's configurator was among the best we encountered.