Will it kill the powerbook

After upgrading the ram in my macbook...
I have now got a spare two 512mb sticks of ram
I have offered the ram to a family member who is willing to have them.
However after checking crucial.com
although both sticks are DDR the speeds are different
the powerbooks ram speed is 4200 whereas the macbook is 5300

What effect would fast ram have?

You mean PC2 4200 and PC2 5300? That is DDR2 RAM. What is your family members Mac? If they both use DDR2 RAM the faster should work. Done it here on my iMac and not an issue.

My iMac uses PC2 4200 and I put in a stick of PC2 5300.

Yes its DDR2 Ram, sorry i didnt make that clear.

Its the powermac G4 15" screen.
http://www.crucial.com/store/listmodel.aspx?pl=PowerBook&mfgr=Apple&cat=RAM

If its not the lowest DDR2 model would DDR2 work in a DDR memory slot?

No, you can not put a DDR2 in a DDR slot. Will not work.