We've got three 40' x 4' raised beds in the veg plot. They are raised by about 9". I put them in for two reasons, 1/ because it helped even out the soil where the garden has a slight incline and because I got fed up with edging them where the grass kept invading.
My brother has a garden that slopes quite steeply in several places and he used old railway sleepers, stacked three high at one end to make his beds more level. They're a bit pricey but they've been there about ten years and are still solid, so maybe long term not a bad option.
Dom
PS Note to self must get up the veg plot and do something with it.