Why can't an upgraded version of Microsoft Windows XP or a full
version of Microsoft Windows XP that does not include Service Pack 2
(SP2) or later be used for installation?
An upgraded version of Microsoft Windows XP cannot be used because you
are required to validate your copy of Windows by inserting an original
Windows CD, but there is no way to eject the first disc until after
Windows installation is complete and the Boot Camp drivers from the Mac
OS X 10.6 DVD are installed."
More at http://support.apple.com/kb/HT3777
However Microsoft changed the licensing that requires XP Pro rather than XP Home (either works, but it is a bone of contention worth noting) and in Windows 7 Pro you get that XP MODE for the price. And again Microsoft's licenses are quite the topic. I'll excuse myself from that discussion.
If I were to do this, I'd go with Windows 7 Pro 64 bit. But I have my reasons.
Ok so I love my Mac but there are, unfortunately, a whole load of things I miss from the days from when I used Windows. I don't have a hard copy of any windows operating system, so I'm going to opt for a direct download option (of course paying for it, not thepiratebay.org or anything, legitimate copies only). Will this be a problem for when I want to use Boot Camp or Parallels Desktop? I'm not sure, and I want to know what to do for sure before I actually make the purchase.
I will probably only want XP, as I don't want to spend too much, and all the stuff I want will be compatible with it.