Depends what you want to do in SR. For me one day at Angkor enough liao. I got templed out seeing the highlights of the 3 main temples and the surrounding minor ruins so I spent most of my time elsewhere. There's a lot more to Cambodia than just the Angkor temples. Best to treat 3D/2N as a sampler or recce so you know what to go for when you return.
If you need to get from SR to PP, try Mekong Express bus, abt US$9, abt 5 hrs. Nice tour buses like the ones running between KL/Sin.
there are cheap cheap US$12/day (twin bed) roome...
and tuk tuk - 3 days u give about US$25 is more than enough..plus if u sayang the driver, go out with him at night..go to the MUAY TAI RING. and see kickboxing!
it is their HOT HOT HOT season (temperature) season there now...
Air tickets nowadays pretty cheap. I remember when I went there about 7 years ago, it cost me SGD700+ just to fly to Phnom Penh. Then took a 6 hours "under the HOT sun" "above the deck" boat got roasted like BBQ chicken to reach Siem Reap.
An alternative route, which might be pretty interesting for people who have time, take a flight to Bangkok, then a bus to the border of Cambodia, can't remember the town, but it's a casino town, then a 3 hours bus to SR. You might need to stop over at the casino town for a night, but hotel there is cheap. And you got a chance to see the border of Cambodia and Thailand, where mornings hundreds or thousands of cambodians cross the border to Thailand to work or for business.
There are alot of things to see in Cambodia. Flying there for a 5 day trip to really enjoy Siem Reap is not enough. Take a 3 or 5 day pass to the ruins and take your time to walk though all the temples, understanding the history why and how the temples were built will be a very worthwhile history lesson. If your aim is just to shoot the sculptures and sunrise, sunsets, then 2 day pass is enough. But there is much more to a place which has about 500 years of history, which was once one of the most powerful kingdoms at it's time.
There is a town called Batabong (I think that's the spelling...), pretty nice and quiet town, where you can take a river taxi from there to Siem Reap and see the boat people living along the rivers.
Get hold of a Lonely Planet and plan your route from there. In all these years, I've never used the local agent to book my trip. Calling the guest inns listed in lonely planet and using google for search is a good tool to get hold of your accomodation done.
Best time to visit Cambodia is year end and avoid monsoon. But if you are a seasoned backpacker, anytime of the year is good. You get to enjoy different aspect of the country at different times.