You'll be fine.. Just boot off the Win 7 disc and start the install process as you normally would than once your at the partition manager just select the HDD you wish to install 7 to then let it install.. Once it's done you'll encounter the Boot Manager each time you boot up your machine.. This is where you choose which OS you want to boot into...
The fact that one is 32-bit and the other is 64-bit won't be a problem..
Your XP installation should be fine, though it's always a good idea to do a backup of all your personal files JUST IN CASE..
No you don't need to change any settings in the BIOS to enable dual-booting..
The only things I'd suggest doing prior to the installation is take the time to research your hardware and make sure there are 64-bit drivers available for each device or you could be regretting the installation.. Also, make sure each app you regularly use is 64-bit compatible to avoid any headaches after you have 7 installed..