Updating to Mac OS Leopard on a partitioned iMac?

Hi, I have an a 24" iMac (the older, white one) and its currently on OS 10.4x Tiger, and it has a boot camp partition for Windows XP. I want to update the mac side to 10.5x Leopard, and later to Snow Leopard, but, firstly I am unsure whether the everything will be wiped, and secondly, if all my mac files stay, will the Windows XP partition be affected in any way? Do I have to back everything up or is there no need? Please help! Thanks.