I do recommend Nagworld as a place to stay. I've stayed there my last 5-6 trips to Phnom Penh. I don't gamble much.
It's got a 90% Asian clientele, but the rooms are nice enough, beds comfortable, flat screen TVs, good strong showers, wired internet, good lobby and facilities, etc.
You can get rates for about $95 a night at agoda.com or expedia.
The place has a bit of a bustling, "entertainment" atmosphere at night with the casino and nightclub and whatnot, but the noise doesn't reach the rooms. A lot of dolled up women walking around the place at night too. If that offends you, stay away.