In any case, this looks to me like a bug. It's not reasonable for Evo to fail to index (or expunge) on a 4GB machine. My desktop has only 2GB and I've never seen this (my local Inbox is currently 130MB). You should probably report it to http://bugzilla.gnome.org.
This sounds like http://bugzilla.gnome.org/show_bug.cgi?id=559153, although the symptoms are a bit different. I am not sure about Fedora's status (I only have time for Ubuntu), but hopefully someone there is building the test packages for this bug. Bear in mind, the patch is still in flux, and tests are required. ..hggdh..
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