I'm gonna guess overclocking is sorta new to you. You should read few guides on overclocking the 775 socket.
However, I can tell you that you can probably get it to 3.6Ghz relatively easily. Also you would have to have better cooling than stock cooling.
Another easy way to get a "mild" overclock is using the ai (automatic) overclocking features of that board. The p5n-d should come with "cpu step-up" or "ai overclock" feature in the bios. This is the easiest overclock ever (if and when it works, usually it does)
To set up a automatic overclock:
1. Go in to the bios setup
2. Got o the Advanced page
3. Select and enter "JumperFree Configuration"
4. Select "AI Tuning" and set it to "AI Overclock"
5. Go down to "Overclock Options" and set that to your desired percentage (5, 10, 15, 20)
6. Save settings and exit the bios.
7. You should now be overclocked, use cpuz to verify the frequency increase
PS: Don't let speed step fool you. Put the computer into load (use something like intel burn test) to make sure and see your real frequency. Also use intel burn test to verify stability. Alternatively you can turn off speed step so you always run at the set frequency and it will never throttle down.