Although I am not certain,iPhoto should be OK once you upgrade because on the Mac OS X operating system upgrades most of the time replace only the old files which would mean say MAC OS X 10.5 to MAC OS 10.6.
Your personal files and settings would remain the same. The only thing that would change is possibly some interface tweaks the OS / App upgrade contained and/or some other added features or in more simple english,
If you Upgrade the software the old version would be replaced by the new ones meaning documents, Apps, and other files (including iPhoto Libraries) would remain untouched because they were not part of the APP and / or OS.
Mac OS X 10.6 Snow Leopard is a really good operating system and if you haven't already upgraded you might want to consider it.
Now you should check if your computer is intel based. an Easy way of doing that is by recalling if you purchased the computer 2006 or after.
Even better if your intel processor is part of the core i series (e.g. core i5, i7 etc) you will be able to upgrade to the upcoming Mac OS 10.7 lion os instead of 10.6.
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Hope this was helpful