obviously you have to limit yourself to 2-3 destinations with only one week in cambodia. unless you are absolutely not interested in temples, siem reap for the amazing tmples (angkor wat, angkor thom, ta phrom, banteay srei etc.) is a must. for how long depends on you. some have enough after a day or two, others (like myself) dont get tired of it.
the second must (in my opinion of course) is pnohm penh, for the more recent (and tragic) history of cambodia. also for markets, people, food and a lively atmosphere.
if you want to include a 3th destination (would be too rushed for me), i think it would be battambang. less touristy than the 2 other destinations, a great for scenery and people.