Depends on the type of games you play, but typically e.g an FPS game does/should do its damnedest to avoid using the HD during gameplay. Assuming there <i>is</i> enough available RAM for the game, of course. If not, adding more RAM is by far your best bet. A faster HD would only affect loading times, not the gaming experience itself.
Again, don't know which games you play or how much RAM you have, but if gaming performance improvement is what you're looking for, I think it's safe to say your money is much better spent elsewhere. More/faster RAM, faster CPU, better video card, better sound card... practically anything but the HD.
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<font color=green><i>Feet are frozen, hair's on fire, on the average, everything's fine.</i></font color=green>