It'll work, but you theoretically would get better performance (about 10% difference) with matched 1GB modules in RAM and graphics-heavy applications.
But, if you're using multiple smaller apps at the same time, then you might be better off with 2.5GB - the extra 512MB might prevent OS X from swapping out to virtual memory.
So, basically, it depends on what you do with your machine. But either way, it'll physically work.
Edit: WAIT! You have a Core Duo MacBook? Not a Core 2 Duo? Then, NO, it wont work at all. Those machines have a 2GB maximum RAM limit. 1GB per slot. I've read that they wont even boot with a 2GB DIMM (let alone two) in the machine.