How would the base Mac Mini compare to a 2008 iMac?

[Star Nuts]

I own the one in the middle, upgraded to 4GB of RAM.
http://support.apple.com/kb/SP485

I'm looking at the base model, upgraded to 4GB of RAM. I'll probably upgrade to 8GB, but let's say 4GB to keep things fair.
http://www.apple.com/macmini/specs.html

Will I see a noticeable improvement in speed? With my iMac, I'm seeing the spinning wheel a lot more when moving in and out of programs/pages quickly. I don't do any heavy duty stuff. I use my computer for the internet, light editing in Aperture, and running my very large iTunes database through an external drive.

I was planning on upgrading to an 27'' iMac but I'm realizing I'm spending a lot of money for power I don't really need, though the screen would be nice. But I could just get a different screen for a few hundred. I know it won't compare to the iMac, but I don't think I need it to. I also feel like it'll be a better value in the end if I decide to upgrade again in 3 to 4 years, seeing as I'll only lose a few hundred dollards compared to close to a grand.

EDIT
Now that I think about it, I could just use my current iMac the main display, saving me from buying a keyboard, mouse, speakers, and a monitor.. That's assuming my iMac CAN be used as a separate monitor.

 

[paulrbeers]

Processor - Mini wins
GPU - iMac wins*
Memory - Mini wins (faster, and expandable to 16GB)
Storage - Mini wins (also with Thunderbolt is more expandable)

* I don't think there are any direct comparisons, but I believe it would be faster, but neither are going to be real winners compared to a modern discrete GPU

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EDIT
Now that I think about it, I could just use my current iMac the main display, saving me from buying a keyboard, mouse, speakers, and a monitor.. That's assuming my iMac CAN be used as a separate monitor.

Only the 27" iMacs can be used in Target display mode and even then the compatibility varies depending on which 27" iMac.