It'll slow down. Nothing can mimic the feel of a fresh OS install on a clean hard drive. Fill the hard drive up, use the computer for a few months, and it'll run a little slower.. longer to boot and shut down... and that's probably how fast it'll go until you get rid of it.
Example: I got a G4 mini.. after a few months it slowed down a little (meaning it didn't feel brand new). A year and a half later, it's still on the same original OS install, and it hasn't slowed down any since it lost the clean OS install feel.
Hope this made sense, it's early over here...