as you got leopard , and leopard has time machine , get a external drive , just as big as all files inclusive operating system that are at the moment on your Mac , but i would just get one as big as your internal or even bigger they are cheap now , then do a time machine backup on that (time machine once you activate it will ask you for permission to use that external drive ,but tell time machine to backup the entire drive , when that is done best unplug your external to avoid selecting it on the next step
now just on the internal drive , you can create partitions with disc utility from your install disc one for osx as big as you think you need it for osx and all your apps and and one for whatever you like (own data) , now install osx in this first partition ,
once install is done you could even repartition that still empty second partition in smaller partitions with disc utility from within the application folder , once you think everything is partitioned like you think it should be
plug in your external with time machine backup and just restore from your time machine backup , time machine lets you even select single files to restore for example your pictures , even single pictures after it has restored them you can drag them into the partition of your choice , same for your music or whatever , and the good thing if you made a mistake or you think its not enough space on the boot partition or any other , you can always do it all over again until you think now the partitions have all the perfect size , as you still got your time machine backup , so no data lost