The SQL Server install process will always put about 3GB of files on to the System drive. Most of this is .Net routines that must be stored in the GAC which in turn must exist on the sytem drive. The SQL install process also needs about 2GB free space on the system drive while it runs, on top of what gets placed there permanently.

When you install service packs you should plan to add another 0.5GB to the system drive for each SP. A CU can also add a significant amount to system drive use. We have a standard of a 60GB system drive for any SQL Server machine, to allow enough space for patching over the expected lifetime of the box. We also boot direct to the SAN and do not use any server local drives, but that is just our preference.

The most that you can get to install elsewhere adds up to about 0.5GB. Service Packs hardly add anything to a non-system drive that has SQL binaries. My advise is to install all SQL binaries on the system drive, as this saves you a drive letter that can not be used for much.