I think it would depend on the replacment that you purchase. I have an 02 sedan. I replaced (or rather upgraded) to a Koyo Alumn Radiatior. I didn't need anything else, however the Koyo doesn't have a filler cap either. If the replacement that you get doesn't have a filler cap make sure you "Burp" your upper rad hose to ensure that there are no air bubbles that can get trapped in the system and cause more over heating issues..
02 WRX.. "It's fun when it runs.. Problem is that it hardly ever does..."