A hard drive won't improve gaming at all, aside from shortening loading times. A fast hard drive would absolutely make the computer feel more responsive, but if all you are looking for is good framerates, don't expect a hard drive to help.
Oh, and in that price range, I'd go with either a WD Caviar Black or a Seagate 7200.12.
^Yep, as cjl said, hard drives have nothing to do with framerates, when you open a game, its loaded into your ram so when you play it, your hard drive is isn't doing anything. However a fast hard drive will can make the game open faster and load maps faster.