the PC2700 is only rated to run at 166mhz (333mhz DDR), and the PC3200 is rated to run at 200mhz (400mhz) DDR.
Your Barton 2500+ runs on a 166mhz FSB, so either will work just fine. But if you plan on overclocking, you would definatly want to opt for the PC3200.