About a year ago, I have partitioned my laptops HDD into 3 parts.
First partition contained Vista Home Premium x64, second one had XP PRO SP3, and the third partition contained my data (games, movies, text files and 3dsMax files for projects).
There are no horror stories involved here.
The procedure to have all of this work is quite simple.
First you install an older OS, then the newer OS.
However, in certain situations, the boot manager will become corrupt and will not detect one of the OS-es.
That happened to me, and the only thing I had to do in order to fix the problem was boot the computer with my Win Vista x64 install disc, and select 'repair computer' option.
That fixed the boot manager and it correctly showed Windows XP and Vista right after POS screen (with 25 seconds of time for me to choose which one to boot).
The same boot option would occur if you had XP on one HDD and Windows 7 on the other.
Going into BIOS and changing the boot order every time you wish to boot into a specific OS on a different HDD doesn't come into play (unless you are installing XP on a new computer ... in that case, you have to switch from AHCI to IDE in BIOS in order for the installation to go through, and then simply return it to AHCI once you install SATA drivers in XP).
The Boot manager detects multiple os-es regardless if they are on a different hdd or on the same one.
I personally recommend that people have 2 partitions on their HDD.
One for the OS and programs (roughly 60 to 70GB large) and the other for games, movies, media and general data storage.
You can avoid a lot of problems like that (ranging from virus infections and corruption of data).
Plus if your OS becomes corrupted, you only have to reinstall the OS and programs knowing your critical data is on another partition which won't be affected by the reformat of the C partition.
I've switched to Windows 7 x64 now.
Minor incompatibility issues with legacy software due to incompatibility wit 16bit code, but there are ways around that.
Truth be told, XP is a VERY old OS by now, and I would personally suggest switching to 7 if you can (unless you have a very old system or actually want the older OS for whatever reason).
XP in x64 form was/is an infant form of x64 code.
Very bad driver support at the time, and unfortunately, MS never really worked on it to release SP3 for example in order to improve it (although the SP's in themselves are just a collection of patches readily available via Windows update anyway ... and unless the SP brings speed improvements and optimizations that aren't part of the standard patches, there shouldn't be too much of a problem in this area - so it's not surprising that XP x64 would be stable - provided you have necessary drivers).
What's interesting though was that Vista in x64 form was far superior (which is why I used Home Premium x64 for some time on my laptop - it came originally with x86 Vista Home Premium, so using my OEM key on the same but x64 version was better because I wanted to fully use my hardware).
And ever since Windows 7 came out, I decided to use only that OS (in x64 form of course) and removed the other two (retaining the second partition for data storage).
Not really looking back since I get to use all my 4GB (and more in the future) properly (which comes in handy working in 3dsMax).
The only thing I lack now is x64 Flash and internet browser support.
