The “manual” button during the install just means that you choose the partitions. The easiest way to do it would be to simply locate the 20GB drive in the list and make it one big partition for “/” (root).
It would be better, though, to make at least two partitions, one for “/” (root) and one for “/home” (home, i.e, personal files). That way, if you have to reinstall or upgrade later, you can often do so without clobbering your current home directory.
If you go that route, I’d create root “/” to be about 7 gig, with the rest going to home “/home”. (There are differing opinions on this, and if anyone wants to suggest something better, please jump in.)
I still haven’t decided on a partion size yet… Still backing up data on my drives first and downloading the DVD ISO. My primary concern here is will Suse fine my XP on my 40 GB drive and install GRUB properly, or do I need to do it manually?.
IMO, with a drive of that size, it may not make sense to use a separate partition for /home. You could easily end up with extra space on one partition that you need on the other. You do need a small partition - about 1GB - for swap.
As far as the boot, ordinarily the installation goes fine. However, there can always been factors in the machine unseen by the user that can create complications. If there is a problem though, it is fixable from the command line using DVD Rescue. XP will not be able to boot openSUSE on another drive, so other than using a 3rd-party boot loader, using grub in the MBR is the only choice. If the bios is set up to boot from the 40GB XP drive, then that is where grub will be installed. If you wish, you can change the bios configuration to boot from the 20GB drive and grub will be installed in its MBR instead. Just be sure to decide which in advance and have the bios configured accordingly before you boot the DVD. If the 20GB is connected via USB, that introduces new issues; you need to tell us before installing if that is the case.
If you are booting from the USB, as configured in the bios, then that drive’s MBR is probably where openSUSE will suggest it install grub - you should check that during install. Note that you can also enter the dialog here and manually instruct the grub installation however you wish.
If your experiencing drops, try a different download client or bittorrent. IIRC the download page will try to redirect you to a nearby mirror; you can also choose your own mirror from the list and navigate to its directory for the download.