I can't speak to the Vietnam part of your trip, but I've traveled to Cambodia enough to tell you that you're selling yourself short. Let's consider Siem Riep first. This is arguably one of the most awe inspiring temple sites on the planet. You're only giving yourself two days to see it. Unless you're easily bored, you need three solid days to do this place justice. The best time to see the temples is early in the morning and late in the afternoon. Take up a local custom and feel free to nap in a hammock during mid-day.

There are some absolutely fabulous hotels in Siem Riep, but I recommend against staying in them because you'll be torn between relaxing in comfortable (and expensive) surroundings and getting out there and exploring the temples (which is, after all, the whole point of going there). I always stay in a comfortable guest house at a fraction of the price of a hotel.

In the evening, be sure to check out the Apsara dancing shows. I'm always astonished when I compare the bas-relief carvings on the Angkor walls with the live dancers on stage.

I advise friends to plan at least five days in Siem Riep. Arrive on day one, get settled in to your room and explore the town a bit. In the late afternoon you can buy your 3-day pass to the temples. The pass begins the next day, but you're allowed to go in to see the sunset right away. You can then spend the next three days seeing all the sites. Sleep in on day five before you depart. If you spend more time there, I like visiting the floating village on the lake and checking out the Khmer-oriented culture village (not to mention the (over) abundance of restaurants and bars in the city).

Now, Phnom Penh. I like the city because I can appreciate what the place has gone through in the past hundred years (lot's of run-down colonial buildings). I can't say it's much of a big draw as a tourist destination however. The palace, museum and temples are interesting, but do not justify a trip in and of themselves. I like the city for meeting the locals on the river walk, shopping in the central market and "Russian" market, admiring the architecture and visiting the arts school.