if your constantly running out of ram and doing alot of swapping, it'll speed it up signifigantly.. If your not (and you probably arent) it'll slow it down ever-so-slightly..
Only thing you probably do to eat that much RAM would be editting huge photo files, mastering big ass wav files, encoding dvds, etc..
Assuming you're like everyone else and you just browse the web and play games, I wouldnt bother..
But ####, stick it in, if its unbearably slow, take it out..