A solid state drive (SSD) is the single best upgrade you can make for an HTPC to improve the speed while making the system quieter.
A solid state drive can help you shave off several watts per hard drive which means less heat and noise.
An SSD will make your user interface feel snappier, for example by helping with the loading of 1080p fanarts and other metadata.
As an additional benefit, your system will boot and shut down much faster, and the system will just feel much snappier overall.
SSDs are really light-weight, so even if your case does not have a slot for it, you can use an adhesive tape to attach it anywhere inside the case. This way you can add more SSDs later even if your case does not have dedicated slots for them.