I have Vista Ultimate installed already, however, I was also going to put Win XP on as my second OS.
OK that will be your only problem. XP will wipe out Vistas new bootloader, which, depending on how lucky you feel and the colour of the moon at the time, you may be able to get back by running Vistas recovery tool on the DVD... or, you may have to do it manually using the appropriate command and the command line interface. Sorry, I forget what that command is.
Usually, you should probably have no problem using the recovery tool. But you'd have bmuch more smooth sailing if you installed XP first.
Other than that, the only issues that may arise is when you swap from OS to OS - Recycle Bins and System Restore points are incompatible with each other and tend to get wiped (recycle bins on random occassions, system restore points all the time without fail).
The usual 'don't install both on the same partition' and that's about it =]
it seemed to override my Linux platform,
Generally the XP CD asks which partition you want to install XP onto - if it didn't, your OEM copy may be automatically configured to kill the first partition or something. If that's what happenned to Linux, it will probably happen to Vista.
If you did choose to install to a different partition, then all of Linux that got scrubbed was its bootloader - but using Linux, you should have already known that =] Unless maybe the first partition wasn't NTFS or FAT and XP wanted to store its bootloader there and didn't ask questions before reformatting. I'm a little fuzzy on how that works.
Also, bear in mind that Vista requires a fair chunk itself (10GB on my system, where XP takes up less than 3GB).
15GB is needed during the install procedure and MS recommends you give it 40 ;]
Pre-installed and no disc in the package...
No OEM disc? No recovery? Sounds to me like you got a raw deal worth complaining about.