Would I see much performance degradation going from a 2009 Mac Pro (2.66 GHZ quad, GForce GT120, 12 GB) to a 2012 i7 mini (either 2.3 or 2.6 ghz quad)? The new Mac Pro is really overkill for me and I would like to sell my Mac Pro and reclaim the space it takes up. I don't do anything too intensive. Maybe a little Photoshop CS5 for photography and iMovie for video editing. I would take the hard drive I use for movies and videos I watch and place it in either a USB3 or Thunderbolt enclosure.
Whilst as others have said you could take your 2009, flash update it to a 5,1, SSD, CPU upgrade to a W3680/W3690 so get a 6 Core 3.33/3.46 CPU expand the RAM out more then a mini could take, I get the impression that this isn't about performance issues.
If what you are doing now isn't taxing your 2009 Mac Pro then the 2012 i7 mini should be able to cope with what you are doing.
I am guessing that as you say a little photoshop that we aren't talking professional use, but more of editing the holiday photo's etc, so upgrading the 2009 Mac Pro will be overkill again for you.
iMovie doesn't need much in the way of GPU usage, and Photoshop CS5 again isn' t going to need the GPU much either.
I suspect that there were quite a few people back in 2009 who bought Mac Pro's as there was a big difference between the mini/iMac/mbpro and the Mac Pro's at that time. Now that we have seen the prices for the Mac Pro there will be probably be quite a market for people wanting 4,1/5,1 boxes, rather then the new ones.
The Current Mini when compared with the 2009 Mac Pro for Light Use will cope perfectly well. You can reclaim the space that the 2009 Mac Pro takes up, and you can also reduce the Power Usage