I wouldn't pay $80 for just a speed upgrade. If it were only $20 I'd jump on that, but the difference in responsiveness/performance isn't worth $80 at all. You probably won't even notice the difference since commonly used files will end up being cached anyways and programs you really use a lot will probably be on the ssd anyways.
Just a note, putting games on your hdd (as opposed to ssd) won't give you any performance boosts (maybe 1-2 fps) - it only affects loading times. The difference in loading times between 5200rpm and 7200rpm won't be that big either.