I presume you are doing a custom installation.
This is from memory, so subject to correction: choose sdb and remove the
existing FAT32 partition(s). Choose add, 'use this partition as root' ('/')
and use ext4 as the filesystem. It's a good policy, but not essential, to
also make a home partition. Root can be 10 to 25 GB in size and the rest,
except for a small linux swap partition, is for 'home' which is where all
your personal data is stored. Swap is the same size as your RAM.
I hope this helps.

I had much the same questions when converting to 10.04 a few months
back. I didn't much understand the way Ubuntu presented the disk
partitioning and formating. I downloaded "Gparted", another ISO
you create, and boot from, to show your disk partitioning as it is and
to me seemed easier to repartition in a more graphical presentation.
One repartitioned with labels it was then easier to finish installing
the Ubuntu. It is nice to have the Gparted disk to boot from to check
partitions, and in case you want to do further adding or resizing later.

The Ubuntu 10.04 partitioning/installation screens frightened me a bit, because
the first few did not show hdb, which is where I wanted to install. hda has XP,
which is not to be destroyed (yet).

Above comment gave me enough courage to proceed, hdb did show up, and
the dual-boot installation is now completed, with hdb partitioned with / and
swap partitions, for now.

Two more questions:

1. Above advised having a partition for /home, so I'd like to do that. hdb
is a 40 GB drive, so maybe 1 GB for swap, 15 for /, and 24 for /home ?? What do
you folks do?

2. Does Gparted provide for repartition without re-installing?

Your choices are about right depending on how much information you keep
in /home.
On Gparted, it is a tool that doesn't get installed, you partition your
disk and either format
or not format, label the disk partitions which can make it easier to id
the partitions, I used
it because I didn't feel comfortable using the one on the install disk.
Now that I have been
through the install disk a couple of times I feel better using it. I
have an extra disk here
and will install it in a desk PC once I figure out how to install it on
the other bus and ID it.
and I will probably use Gparted to get the ID of the first disk, then
install the second and
run it again to ID the second, I haven't played with the buses before
and it sounds interesting.

this link will explain how to install Ubuntu (.10. It is the same for 10.04
Look at the part for manual installation as that's how to create partitions. You
should do one for the / about 15 GB, swap 2xs memory and one for /home using the
rest of the available space. Backup your Windows drive before you begin, just in
case.

....softpedia.com/.../...-Ubuntu-9-10-126370.shtml

I didn't do it that way, because, unless I missed it, those instructions don't
describe how to put Ubuntu on hdb, leaving that other system intact on hda.