The first thing you should find out is if you can run the vanilla kernel or you need the voodoo kernel. You dont state your config so its hard to give any advice. If you are running vanilla 10.5.6 you should be able to update to 10.5.7 without using another kernel. If not you would need to do something as what Taisto suggests.
seatbelt.kext usually needs to be kept in sync (same versions) as the kernel to avoid problems
Also if you are using a specific (patched) kext for your hard drive controller you need to be sure it doesnt get overwritten by the updates.