I recently tried to replace the 2 stick of 256 in my sister's computer with 2 sticks of 1 gig. However, after trying both sticks together and separate multiple times only to be met with a long beep, I ended up just putting the old sticks back in.
Both are labeled as PC2-4200, though the new sticks are about half the height of the old ones. They seem to fit snugly, but I know how finicky those clips can be.