1 WEEK IN CAMBODIA

Depends what you want to see and do there.....

It would be enough if you only wanted to stay in one place such as Siem Reap, you would have enough time to visit two places and the travel time between them, but not enough time for three places...
If you really only have a week, I would still definitely go and just make the most of it.

how will you be returning to vietnam?
if overland, no VOA at the border.

As above. If you will be returning to Vietnam after you visit Cambodia then you need a multi-entry visa for Vietnam. The cheapest, easiest visa option I've found for Vietnam is www.vietnamvisaprovider.com for a pre-approved visa letter online. However, this option requires that your first entry to Vietnam must be by AIR. Also see www.canbypublications.com for good information on travel around Cambodia.

why do you want to visit cambodia?

might be able to help if you could provide a little info

I already apply for multiple entry visa, so I shouldn't have any problem once I go back to Vietnam.
In Cambodia I'd like I was thinking to spend 2 nights in PP, a couple of days on the beach and then 3 nights in Siem Reap; this is my idea but any suggestion is more than welcome!
Thanks everybody

How about getting between those places,and to and from Vietnam?

If you take out travel time,that is too fast for my liking....I'd drop one (personally,the beach)

We are thinking about booking a bus from Ho Chi Minh to PP, it takes around 6/7 hours; then from Siem Reap to Hanoi our idea is to take a flight and save time.
I've seen there are a lot of bus companies to travel within Cambodia, would you recommend any of them?

Not really long enough but you could go to Kampot from Mekong then PP then onto SR. Allow 1 day travel time between PP and SR and 1/2 day between Kampot and PP. Beaches not that special in Cambodia. PP Sorya is the bus I used most, fairly reliable but some buses are old. If you want post bus then there are several like Mekong Express.