Since it's a visual thing, cost will be your biggest factor. Where will the dirt come from and what type is available. Clay does real well at holding itself together in the rain, loam will rut out on you fairly quckly. If you want to plant trees on top of it, you will have to make it wide enough on top so that the trees can establish a strong enough root ball to support themselves when mature. If you are planting the trees behind it, you need to decide if it's something you want to maintain by mowing it. A 3:12 pitch, as in a house roof angle, is pretty steep for side hill mowing, but doable. When you get steeper then that, it gets dangerous, but I don't know what the limit is. You might be fine with a 4:12 pitch, but I think that would be the max.
If you just want to let it go native, and you have clay soils, you can make it just about as steep as you want it. If compacted, the clay will hold up pretty good.
Schmism has a good point about free dirt. If you have anybody doing dirt work in the area, all you need is a sign saying you want "clean fill dirt" and another sign showing where you want them to dump it. I'm always amazed at how a sign will show up and then the dirt will start getting dumped there.
If you are bringing it in yourself, or digging it from another location, distance and the size of your equipment will be the decideing factor. 500 feet is a very long way to build anything!!!
Have you seen a berm that you thought was attractive? The ones I've come across look like berms. Nothing that I'd want to use myself, but for sound, they do a good job. Look up Thuja Trees, I think you might be suprised at what you can buy and plant that will give you a very nice screen.
Eddie