There are different aspect ratios; 1.33:1 / 1.78:1 / 1.85:1 / 2.35:1

Each will have different pixel settings on your video presets, each will require different bitrates for the best output...

At 320x180, thats the same as the 1.78:1 ratio, or very close. You'll end up streaching 1.33:1, and squishing 1.85:1, and 2.35:1. do the math for each ratio, and you'll end up with this information.

1.33:1 (4:3) = 240x180 Pixles (Regular TV)
1.78:1 (16x9) = 320x180 Pixels (HD footage)
1.85:1 (Acadamy Flat (Flat)) = 320x173 Pixels (Some Movies)
2.35:1 (Cinimascope) = 320x136 Pixles (Most Movies)

All these ratios will ft nicely onto our screens.

If you adhere to those settings, and keep the bitrate as high as you can, while keeping physical size down to whatever your requirements are you should have very nice movies, that still look right! Remeber, if you want the movies to look great, pump up the bitrates, but a 2-Hour Movie, set to the MAX might take up more than a Gig!