they do help but they do sacrifice some cooling as well, here are some tips
a top fan adds cooling but it also adds some noise, depending on where the computer is, but it is not a deal breaker. a good case will have a top fan
a side fan will be very noisy depending on your GPU, so avoid this
also, placement of the computer is very important
most desks are going to be near a wall, that is just the way it is
but having the computer vents in the back facing a large open area cuts down the noise considerably
having an aftermarket CPU heatsink also reduces noise
I use an antec P182
if you plan to buy an antec, just make SURE you do not get the number series, 900, 1200 etc.
coolermaster makes some great cases, you happen to have the budget model that mainly adds cooling
your hard drives really make more noise than your 4850?
what you might try first is
are your hard drives in the lowest bays of the computer?
block that lower side fan with a piece of plastic
normally, I would say the loudest thing in an OEM computer is a cheap PSU that has a noisy fan
then, the CPU fan reving up constantly
then it would be a hard drive, though, you can always hear a hard drive spool up, but that only lasts a few seconds