This is not working in Opera: you cannot block Flash content by right-clicking on it. You can only do that by right-clicking on a non-multimedia part of the page (like text or background), select “Block Content…”, then click on the Flash or image you want to block. You cannot block Flash globally in Tools –> Preferences –> Advanced –> CONTENT (this was left out by Sid) –> Blocked Content, or at least I don’t know how could one do. But you can edit site-specific preferences here, in Content –> Manage Site Preferences, and you can as well turn off Plugins for a site. Or just right-click on a site (non-multimedia part), select Edit Site Preferences, and you can do the same for the site.

There are User JavaScripts that are far better alternatives for globally disabling Flash, but being able to enable them manually by right-clicking on the wanted Flash item. Like this:
http://my.opera.com/Lex1/blog/flashblock-for-opera-9

“Reduce the amount of visited pages in history : (..) reduce the addresses to 100 or 200.”

You cannot enter the number manually, you can only choose from the pull-down menu, and the first two entries is “0″ and “1000″. But you an edit the Opera.ini setting sile by entering opera:config in the address bar.
You can enter these as links to the address bar:
opera:config#UserPrefs|MaxDirectHistoryLines – set the number of remembered typed-in URLs (default is 200).
opera:config#UserPrefs|MaxGlobalHistoryLines – set the number of remembered visited URLs (which are viewable in History and changeable from Tools –> Settings –> Advanced –> History –> Addresses).

Also, it’s a bit old, don’t you think? And for Opera 9.50 beta… One would most likely want to tweak and tinker with Opera 10.0 alpha / previews, or at least with 9.6x.

Also a word of warning: the Custom Substring (ISP | Id) option can have negative effects… For example Gmail is broken with this set.