The 20" iMac ...
When upgrading from 2.0 to 2.4GHz, you get a faster CPU, faster GPU, more VRAM, and a larger HD.
Which makes the 20" iMac 2.4 a decent deal, for a base machine.
It upgrade two things you would want to update, the GPU and the HD.
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However, for just normal stuff the entry machine isn't bad -- though a Apple HD update to something bigger would be called for.
RAM can be updated for $30-100 from a 3rd party depending on whether you want 2GB or 4GB -- and 2GB is generally enough unless you run Pro apps.