Well, of course the multiplier is locked. The only current chips that are unlocked (fully) are mobile Athlon XPs and Athlon 64 FXes. Regular A64s are also unlocked downward.
If there's no setting you have to tweak to... oh, wait. You're using PC2700. Your Extreme Edition is 800FSB, and since it is the Pentium 4 architecture, you may already be killing a significant bit of your performance by not using PC3200. If you overclock that (not sure how well the core with 2MB L3 cache does at that), you'll want something that can go even higher.