You can run the 32-bit or 64-bit version because you have the right processor. So you can download the i386 installer (32-bit) of the AMD64 installer (64-bit).
You can run the installer from the command line of the CD or by booting into a Live version of the OS and using the graphical installer. The graphical installer is easier, but both of them can repartition that hard drive for you. You might want to read through the instructions on Ubuntu's website. Additionally, you may want to wait another ~10 days for v8.10 to come out (with an updated installer).
Why delete Vista? You can just dual-boot. That's what I do - XP Pro for those few programs I still use that need Windows and Ubuntu 8.04 for daily use.