Yeah, the old RAM is DDR PC2700, whereas the new sticks are DDR2 PC2-5300. Wrong interface type.

I have two sticks of 256MB each of PC2700 here, yours for the cost of postage (cheap!). But not as large capacity as you might be hoping for. You can add one of those to the 512MB already there, for a total of 768MB.

Cheers