The 1.83 GHz C2D you have is a Merom T5600, that's way pre-Penryn (the 2.26 you mentioned is P8400 Penryn but it won't work in your Mac mini, it won't even fit actually).
To upgrade you need a socket M Merom. The best you an do is a T7600 (2.33 GHz). These CPUs were EOL'ed in March 2008 so it won't be easy to find one (at a decent price).