Unless you got the 2500+ before the end of November last year or bought it used, it's likely it's locked from the factory. All 2500's made after production week 39 are locked.
The Nforce2 boards that change the multiplier of unlocked Bartons in the BIOS. So an easy way to check if yours is in fact locked is to increase the multiplier (the default is 11.0x) and see if the clock speed increases as well. Otherwise, if it stays the same, the chip is locked (and I am 99% sure it's locked anyway).
Since it's locked, the only way to overclock is by increasing the FSB. The clock speed of a chip is determined by FSB times the multiplier. At default speeds, a 2500+ runs on a FSB of 166 at a multiplier of 11.0....
166 x 11.0 = 1830mhz
Since the multiplier likely can't be changed, you have to up the FSB. Hopefully, you'll be able to hit 200 x 11 = 2200 (3200+ speeds). You may have to bump the voltage to the chip in order to get it stable at the higher FSB. The default is 1.65v, but bump at 0.05v increments (1.80-1.85v max) to improve stability.
Also, I see in your signature that you're running PC2700 RAM, correct? As PC2700 is only designed to run at 166 FSB, it may hinder your overclock and may not be able to hit 200 FSB. You can also raise the voltage supplied to the RAM if the default isn't enough. The default voltage should be around 2.5-2.6v, but you can increase it to 2.8v as necessary. Kingston is a respectable manufacturer and you should get good results from the RAM.
Good luck!