If you have enough room, and the software to create a partition, that is the easiest way to go (or make a new Library on an external HD), though Steam will still take a ~long~ time to run through all of the games you move. The other option is to create your own folder in the current partition and move the games yourself while NOT logged-in to the Steam app on your computer. Then when you do log-in to Steam, it will update its internal settings to your new Library folder, which ~still~ takes a long time to go through. Very painful in either case. The best source for help is the Steam Support Knowledgebase article
Moving a Steam Installation and Games. Basically speaking, no matter which way you go, Steam will end up re-installing everything you move. If you have that partition and create a new Library folder, you'll only have to move what you want (SH3 or other DirectX v9.0c games) to the new Library folder. With a single drive, it ~ALL~ has to move - every last game you have has to follow to that new Library folder you create...
