I personally prefer to do linux first... Ubuntu has an easy partition manager included (gparted) - just format some for linux (ext), NTFS for XP, and NTFS for vista. I like linux to be "in charge" of my machine only because I feel like it's the most stable, even if i don't use it the most. I also prefer the GRUB boot manager to any others. I pretty much have the same config as you're planning on doing so I can just give you my menu.lst file which just has all of the data needed to boot each OS.
As for which distro, Ubuntu is definitely the easiest in my opinion with auto updates, drivers, etc. If you choose a more system-specific customizable distro like slackware, gentoo, or debian, be prepared to do a lot of configuration (but with good results with performance). You mentioned just a bunch of different variations of ubuntu, so between those I would choose Ubuntu. Kubuntu just has KDE, which, if you're interested in, you can just apt-get through ubuntu (although IMO ubuntu runs better using GNOME), and i dont know much about studio ubuntu, but again, id just go with the most popular release; it's always had what i needed (and have fun playing around with its desktop effects
)
You might want to add another partition too if you ever get interested in fooling around with installing mac leopard on your machine - it can be a fun project and if you're tribooting... why not quad
Even if you dont use mac, its good to have one FAT32 partition or NTFS (linux will like it but not as universal as FAT32) for documents/music/etc to use for all your OSs (you dont wanna copy like 40 gigs of music for each partition)