I encountered this with Opera 6.11 and Mandrake 9.0...it took installing windows ttfs and playing around quite a bit to make it look good.
However, when I upgraded to 6.12, the fonts looked great right out of the box! (Now I'm up to 9.1 and reinstalled Opera 6.12 and they still look good).
Could be you just need to have your Anti-aliased fonts and your true type fonts set up before the install...
Or, and this is a shot in the dark...I moved from the shared libraries version of Opera to the Static libs version...could that make a difference?