fastwrites allows the video card to communicate directly to the processor without going through the system ram. there's less traffic so you'll normally see about 3-5% gain in performance (alot of things can cause different results, i usually get 3-5%).
the downside is a lot of motherboard/video card combinations are not compatible together with fastwrites so you'll get crashes and other video errors (that's what happened with you hippopotumus)
sideband addressing is very similar, you wouldn't use sideband addressing and fastwrites together.
Quoted from john240sx