Yes, you need a visa, which was $20 when I was there.  I've never traveled there whilst living in China so I don't know about UnionPay, but USD is universally accepted. You should be able to survive using English, especially for a week.  Just learn thank you, aw kun iirc. 

I was there for about a week, as well.  I entered from Thailand and exited to Vietnam.  Spent a few days in Siem Riep and and a few in Phnom Penh.  Your obviously going to spend a few days at the Angkor temples.  There's not just one, there's a ton, so you could spend a great deal of time there.  Research what you think you'll find interesting.  In PP, do spend time at the s-21 prison and the "killing fields."  Somber, but worth your time to experience.  Lodging is cheap and so is food. 

Have a great time,

                          Jealous

=> personal experience : I've been in Cambodia during the Golden Week, we spent about two weeks (wife + me).

=> touristic areas : The really interesting places which are easily accessible are also touristic. The Eastern ranges are said to be really nice and not touristic, but it's also a bit of an adventure to get there, 1 week won't be enough. ATMs were no problems, even Union Card works ! Dollars is the de-facto currency, you might bring some with you. Local money is just used as small change.

=> Visa : Visa can be obtained on arrival at the airport, but you will save time and a bit of change if you apply for an e-visa.

=> English, do you speak it motherf*** : No problems, it was really good on this aspect.

Our trip

* Phnom Penh, 2 days : okay for one day or two if you are not familiar with South East Asia. You can visit civil war site, but be warned, it's grim.

* Sihanoukville, 1 day : just a stop over to organize your stay on island. The touristic street is a Western enclave of some sort., very cliche. But the people working there are very helpful. Night bus connection to Siem Reap, with buses you can actually sleep in.

* Small isolated island, 3 or 4 days: (Koh Something-Not-The-One-Everybody-Goes) : it was awesome if you are into diving, living almost naked in the jungle/beach without running water and without electricity. We loved every second of it. If you go an island with electricity or Internet, you will miss something. I can give details on private message.

* Siem Reap: 3 days, Angkhor Wat. It's mind blowing, but like eating only ice-cream for 3 days, you get stuffed. Even if the ice-cream is excellent. The floating village thing is government-run scam, it's expensive and none of the money helps the villagers (been there a couple years ago too, no changes despite the huge changes in price). Avoid.

* Battambang : 3 days. Poor countryside (it makes Guizhou looks like freak'in Las Vegas in comparison), with a nice old temple, and a really trashy Buddhist mountain temple site. We named the mountain "Trash Mountain". The old temple 20 km away is nice however, Indiana Jones vibe.We rode a rented motorbike and went around in the dirt tracks in the countryside. Slow life. Nice folks.

* Phnom Penh : 1 day.

I have been to Siem Riep in May with my wife. You can get 'on-arrival' visa. We took China Eastern flight from Kunming to Siem Riep. For a single person the price for a flight was around 3000 RMB (two way). For 2 persons it was 2990 RMB total, two way. You can use ATM there using Union Pay Card. Still, we change RMB to USD in China as we pay less for more USD in China. If you like to bargain it is better to have some local currency because with USD it is either a dollar or half a dollar. With local currency, you can save some money. Though it may not be much but it gives good feelings.

If I remember correct, the fixed exchange rate is like 1 USD = 4000 KHR. So, in terms of bargaining, for example, for 1 USD asking price you may go for 3000 KHR.

We hired a good bicycle @ 4 USD/day. If you like cycling then you can finish most of the touristic area of Siem Riep (those super crowded temples as if reminder of being still in China), in one day.

Have a map handy which you can also get it from your hotel. Siem Riep's main city area is not that big. Restaurants are bit expensive. However we found one or two just on the peripheries of the core center with similar food but cheap price.

All in all, for 4 nights we spent like 500-600 USD which was super awesome. I felt like I was a king. About bargaining, we just did 2-3 times for tuk-tuk (for fun, so not necessary to do it) and while buying some gifts (this is must, and start with at least half of the asking price)

Other than Siem Riep, I have no idea.

By the way, personally, cooking class was just a waste of money. I simply learned it at my hotel for free. I only paid for the khmer food that I prepared myself under the guidance of kitchen staff.

Have fun

-On arrival visa works fine for Americans, course this was 6 years ago.

-I used an American credit card when I was there, not a Chinese one...

-English is well spoken by all tourists, and you'd have to find a pretty crazy place where nobody speaks English.

-Lots of scams, volunteer scams, fake NGO's, beggers, etc.

-Battambang is fun. Messed up but fun.

Angkor wat is worth  to stay for one week if you are interested in photography/ Buddhism/sculpture.

Hi,

You can get the visa online pretty easy so i would recommend to do that, as saves you having to queue just after you get of you flight.  Also don't get a taxi its  rip of from the airport, forget how much but around $8 to get you to your accommodation in Pheom Penn by a tut tut.

I thought Ankor Wats 2 days is enough as once you seen one big temple you sen them all- but up to you.

Pheom Penn its recommended  to see the prison and learn about the history.

I also wished i took more cash as ATM charge high rates sometimes and one one of the places i booked only accepted cash.

Last advice is when traveling between places by coach go for the more expensive bus company's as we went on a more local one and was terrible with to many people on the bus.

Tuol Sleng is a trip....