Xp will take the next available letter and its position on the drive has no bearing on that.
If you have data on the present C partition then the registry has all of its paths set to C for data or programs on that partition.
With XP you can change the drive letter easily but it is time consuming because each drive that you change has to have all of the registry entries that referred to its old letter changed to the new letter.
You can automate it to an extent by using programs like Drivemapper from Powerquest but is it really worth it.
The only recommendation that I would make is that from a new disk situation, once you have installed the OS on the first partition then you should change the drive letter(s) of the CD drives to Z (and Y if you have two) because each new partition that you subsequently make would otherwise have pushed the CD drive letters back by one with all sorts of problems within the registry. The work in the registry is minimal if done as soon as the OS has been installed.
Sorry I could not be more helpful.