I plan to start iPhone Dev. Is a Mac mini 2.26 GHz with 2Go memory powerful enough?
I have mac mini with such processor and only 1GB memory. This is not bad for developing. With 2Gb it will be OK too.
I do regular OS X development on a two year old Macbook, which is more than sufficient. Although I would recommend putting in more RAM. I bought 2x2GB for ~150 USD, which really made a difference. Just make sure that it's actually somewhat easy to replace your RAM, IIRC it's quite difficult on some Mini's.
Update: There are some good picture guides on iFixIt - and if we look at one for the A1283 it seems pretty difficult, put possible.
Certainly powerful enough. A 7200rpm drive is a cheap improvement, a SSD much more expensive but also more dramatic improvement. Your mini may not be capable of accepting 4GB RAM depending on generation but if it is, it's a cheap sweetener.