This is just in case somebody out there doesn't know about this yet but my personal experience with the two major providers in Cambodia leads me to share this with you.
Mobitel still beats everybody in Cambodia with their latest low-cost high-speed offer. You get 1 GB in month for just $5. Nobody can beat that. If you happen to own a 3G phone you get really fast speeds - 460.8 Kbps. Watching videos, however, is still an arduous task. This speed is still too slow for streaming.
The one drawback is that Mobitel covers only three areas with 3G - Phnom Penh, Sihanoukville, and Siem Reap. Outside these areas it drops down to 105 or so Kbps. The price, however, is unbeatable. Even if you exceed your bandwidth you just buy another one-month subscription even if the previous month is not up yet.
Metfone is the next largest competitor. They practically cover all of Cambodia with their cable provided connections; you can get speeds from 128 Kbps up to 2048 Kbps, but it comes at a price. The slowest connection will set you back $50 a month, the fastest $510.
You take your pick.