I'm on a Mac and having video issues!

Open iMovie with the video you have...

File -> Export
Click QuickTime button on top
Compress movie for: Expert Settings
Click Share

Export: Movie to QuickTime Movie
Click Options

Click Settings...
Compression Type: H.264
Frame Rate: Current
Key Frames: Every: 24
Frame Recording: checked
Quality: High
Encoding Best Quality (multi-pass)
Data Rate: Restrict to 500 kbits/sec
Optimized for: Streaming

Click Size...
Dimensions: Custom: 480x360
Deinterlace Source Video: checked

Sound: Settings...
Format: AAC
Rate: 48.000
Quality: Normal
Target Bit Rate: 96

Prepare for Internet Streaming:
Fast Start

...and you've got near perfect web streaming quality!