Yes, make a copy of any data you want to keep from the 10.5 partition, remove it in disk utility, increase the space of the 10.6 partition to use the space that was 10.5.
The 10.6 partition will retain all its data, you will lose anything (that you haven't backed up) from the 10.5 partition.
As always, backup both partitions though, incase you go wrong somewhere!