I think the direct3d9 plugin nowadays is better than opengl one...if you would use win 7 x64 you can rich higher framrates..but remember: update dolphin to the latest revision every months(or week).
You can dawnload latest revisions (x86, x64) from this link: http://www.dolphin-emu.org/download.html
You can dawnload latest revisions (x86, x64) from this link: http://www.dolphin-emu.org/download.html