I own two sites . One is php based file sharing and the other is XF ( this one ofcourse!

my other site uses php's APC Cache to cache the php files & i asked my host to tweak the DB to make the server fast.
What sort of Caching software can i install for this script ? (XfileSharing)
Also what tweaks can be done to keep the server speedy....
(Im keeping dl.cgi because i want to restrict speeds on my 100mbps line or is that a bad idea to enable direct links ? )
Thank you Sibsoft