This response probably answers your question:
Generally speaking, you shouldn't bother with a separate
/boot partition unless you're running multiple Linux distributions
The Ubuntu installers for both the desktop CD and server/alternate CD
have the ability to install over an existing system, preserving your
home directory (and the local system driectories:
/var/local). This functionality also reuses the
user ID and group ID of an existing user, if it has the same username
as the user you're creating during installation.
To use this option when installing, choose the option for advanced
partitioning, then select your existing
/home partition. In
the box that appears, make sure the filesystem selected matches the
existing filesystem of that partition, and that the format box is not
checked. Proceed as normal through the rest of the options.
In Ubuntu 10.10 we had hoped to add an option to the installer that
detected when you had an existing copy of Ubuntu installed and offered
to replace it with the newer version you were attempting to install
(using the aforementioned functionality behind the scenes). While it
did not make the final cut, it is likely to arrive in Ubuntu 11.04.
As for a separate
/boot partition, that's a relic of hardware
constraints of the past (the bootloader 1024 cylinder limit). I can
think of no practical advantage a separate /boot would have on a
modern system, and if not given an arguably excessive amount of space,
it will potentially fill up and create problems of its own, given that
Ubuntu does not automatically remove old kernels.