I was thinking about doing a clean install on my Rev B Imac G5 I have the restore disks that came with the computer and I wanted to know if marble blast gold would be installed along with tiger.
A clean install won't save that much space unless you specifically don't install (uncheck) extra languages and don't install the the developer tools. You might get a little temporary extra room related to preference and cache files, but they will end up rebuilding themselves over time anyway.
The other issue is free space on your hard drive. OS X will choke if you don't leave sufficient free space on your boot drive (or partition) so it can write swap files as needed. If it looks like you're really gonna stuff your iMac's drive and it gets to be more than 80% full, you might consider getting an external drive to store your vids.