Thanks both for your replies. I plan to use the system for programming and photo processing (Gimp, panorama stitching with Hugin/enblend, and some HDR). I am aware that the two systems aren't even in the same league: not only is the Core 2 Duo 1.83 GHz in Mac Mini lower frequency than Q6600, but it's the mobile version of Core 2 Duo (T5600), not the desktop version (E4400, if it ran at 2 Ghz). Actually, all the parts in the Mac Mini are laptop parts, probably to keep heat in manageable limits for such a small case, and the noise very low.
On the other hand, I think I'm currently more limited by the insufficient memory I have than by processor speed. I'm tempted by the lower version of Mac Mini with 2GB RAM - I hope it would be fast enough not to be annoyed by it, and that 2GB RAM would be enough for the photo processing I do (already calculated, Gimp needs about 7 bytes per pixel for a layer with mask, and I have a 6MP camera). Of course, Gimp would greatly benefit from the 4 cores, many of the time consuming functions are SMP-aware, and can spread the calculations among multiple processors. But, since I won't be doing this all the time, I'd settle for a Mac Mini, if it's quieter (I live in an extremely quiet place - at night, I can hear the ticks of my electromechanical watch in the same room). I don't want absolute silence, but something quieter than the ArcticCooling T2 case and ArcticCooling "silent" CPU fan I have now. Of course, if a Q6600 w/ 4GB RAM in an Antec Solo can generate less noise than a Mac Mini, then I'm all for it!