|I know nothing about SUSE anything.|
Are you dual booting XP 32bit and SUSE 10.x now?
As in, do you get the choice of which one to boot with when you boot?
If you are, then SUSE 10.x has the built ability of dual booting XP 32 bit and SUSE, and if you install XP 64 bit, then SUSE 11.1, you should have a triple boot system.
But.... each operating system must be installed on a partition with no other operating system on it, and you may need to re-size existing partitions smaller in order to free up the drive space to do that
Do your two hard drives presently have partitions that take up the entire physical drive, or is there un-allocated (not partitioned) space you could install XP 64 bit on?
Most people DO NOT need more than 4gb of ram.
Ultimate Memory Guide
How Much Memory Do You Need? etc.
Why do you think you need XP 64 bit? Most people don't need it and would not benefit from it, and you may have to get 64 bit versions of some programs you want to use.
Dual booting XP 32bit and 64 bit is easy. You simply install XP 64 bit on a partition that has no existing operating system and it will automatically set up dual booting XP 32 bit and 64 bit.
If you had only Windows operating systems on the drives you wouldn't need to buy anything at all. If you have no un-allocated space available on the drives on which to install XP 64 bit, you can't re-size partitions in XP without losing the data on the partition, but there are freeware partition manipulation programs that are available that can do that without you losing any existing data (though you are advised to backup what you don't want to lose that you can't re-install BEFORE you run them).