Open you dosbox.conf file in notepad and check out the [sdl] section. There's an option in it called "output=". Try out a few of the other renderer options (they're listed in a comment just above it). Type each in and save, one may perform better with your card.
Don't only use the CTRL+F12, try CTRL+F11 too. Sometimes lowering the cycles has actually speeded up the game for me because DOSBox emulated above what my PC could handle at default.
Lastly, type CTRL+ALT+DEL to bring up task manager and make sure there's no rogue applications running in the background munching all your CPU power. You may also want to run a spyware checker if you use Internet Explorer since those can bog down your machine in secret.
Best advice I can give, maybe some of the other users here will have more ideas for you. Your processor sounds plenty powerful.