The CPU speed can be changed by the multiplier or with the FSB. The multiplier can only be changed if the CPU is unlocked (the 2500+ is unlocked for all multipliers of 12.5x and below... the XP1700+ is probably locked if it is an old CPU). CPU speed=multiplier*FSB
Increasing the FSB will also increase the memory speed and with most mobo's also the PCI and AGP bus speed.
Some motherboards also support memory speed increasement without increasing the FSB (often called memory speed or frequency inside the BIOS). This doesn't give much extra performance though because the memory will still be limited by the FSB.
My CPU fan spins so fast that it creates a wormhole