This is where PlayonLinux aids you with boats script but yeah, when you try to do it with wine itself there was some good suggestion a few pages ago

Why not using proprietary drivers provided by Ubuntu?
I decided to uninstall fglrx manually and choose fglrx-updates here, restarted Ubuntu to be sure and voila, osu!'s finally running (a bit slow due to StrictDrawordering I guess, but still)
Edit : regarding my personal problem .. the solution was disturbingly easy ...Espionage724 wrote:
Once you have Wine installed, you first run WINEARCH=win32 winecfg to create the 32-bit prefix (you can close the Wine Configuration window once it appears). Then from there, you run the winetricks dotnet20 command. If it completes without error, you've successfully created a 32-bit prefixIf it mentions it can't install on a 64-bit OS, that means the prefix isn't 32-bit.
Why not using proprietary drivers provided by Ubuntu?
I decided to uninstall fglrx manually and choose fglrx-updates here, restarted Ubuntu to be sure and voila, osu!'s finally running (a bit slow due to StrictDrawordering I guess, but still)