I am a photographer shooting 12MP Nikon in RAW and work with 16-bit tiff files almost exclusively. I have a 2009 3.06 C2D iMac and have no problems at all as far as power is concerned. I also use my 2.4Ghz MBP when traveling and it is about 30% slower but overall just fine. I used PTGui two weeks ago to stitch 11 16-bit tiff files into a panorama and the processing took about 3-4 minutes (my previous XP PC would take at least 20 minutes for this process - and likely much longer.) I also create 32-bit radiance files for HDR in Photomatix in about 30 seconds or less which is pretty good. Of course more power is always nicer but not critical unless you are either gaming, doing video editing or batch processing a huge number of still images which some people may need to do for Event work.
Your files will be a bit bigger but if you are working on one image at a time the i3 would be fine. I would recommend you up the RAM to 8GB which you can do one your own for about $100. I bought my 8GB ram from B&H Photo, Crucial, and it works fine but do not really need it for my MBP. In sum, at least for myself, a i3 would be fine, and slightly better than my current CPU but still photo editing with Photoshop CS4 and Bridge are my main tools, plus the above mentioned programs and
Topaz Labs plugins. If you can swing it the i5 is a better choice but if money is tight then for your stated needs the difference is small, like taking 9 seconds instead of 10 seconds to perform a task. The good news is you can't go wrong with either choice but I would recommend eventually going with 8Gb RAm which is a tiny investment that can make a noticeable difference.
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