I think you are a little confused. Putting certain files on a separate partition is more of a protection thing rather than a speed thing. Normally the /boot files and the kernel are put on a separate partition in order to protect them in case of a system problem. I think you will find when your "pre installed" ubuntu set up arrives it will have a /boot partition. you can use the same boot partition to store backtracks kernel and boot files.
A real install of backtrack needs 2.7 gigs so a min of 4 gigs is what I would recommend. I also recommend this because new linux user of install backtrack and the find its not what they thought it was. Its designed to operate from a usb or live cd enviorment. Its is also a security and penetration testing distro. Its not really good for email and you tube watching. If you are new to linux I would stick with your ubuntu for the time being.