on the K7 platforms it always defaulted to an FSB of 100. So if your CPU was meant to be 133, 166, or 200, naturally when set down at 100 you'd see a loss in clock speed.
With AMD64's all having a base of 200 and the mobo's defaulting to 200, no one has to worry about that anymore.
I fell victim to the same thing before I knew computers. Thought the dude at the computer store did a quick swap on me for a crappier processor or something.
2500+ barton should be 266FSB I believe, so the FSB needs to be set to 133/266 (whatever your board displays it as, both numbers are essentially the same) but it looks like you've already got the problem figured out.