As BlackD indicates, most Source games put the majority of their content in ncg/gcf files in the steamapps directory. As a result, Steam won't let you choose to install these games into a secondary install location - the option never shows in the install dialog. You'll therefore still need to ensure you've enough room for these in your primary steam install location, or muck around with symbolic links.
Although the scythe isn't pre-eminent among the weapons of war, anyone who has been on the wrong end of, say, a peasants' revolt will know that in skilled hands it is fearsome. -- (Terry Pratchett, Mort)
Playing: Killing Floor - Other Stuff