Hi Andrew,
Cuc Phuong is a fantastic reserve, the trees can appear to be dripping with birds at times! One of my favourite national parks within the Orient.
Regarding Cambodia, you are there at the beginning of the peak season for birders, I can offer you a few tips on where to go, e-mail me if you require any further info. A must is to read Frank Rheindts trip report, see link;
http://www.worldtwitch.com/cambodia-birding-rheindt.htm
I assume coming from Vietnam you will be arriving at Phnom Penh first. Try to get to Bokor NP first, an old hill station with some interesting birds, including a Cambodian near endemic, Chestnut-headed Parridge, among other birds (4 days), then to Kratie, along the Mekong, for another near endemic, Mekong Wagtail and the chance to see the endangered Irrawaddy River Dolphin (2 days). It is worth you getting in contact with WCS (Wildlife Conservation Society), who are very active within Cambodia, and it is them whom you need to contact to see Cambodias national bird, Giant Ibis. They are based in Phnom Penh (e-mail me for names and addresses).
From Phnom Penh head northwest to Kompong Thom, to see Bengal Florican and Manchurian Reed Warbler you will need a WCS guide to show you the area. Preah Vihear province next for both Giant & White-shouldered Ibis (5 days). The forest here is quite unique in being very open, and offeres you chance of seeing double figures of woodpecker species, among an array of other specis unlikely to be seen on the rest of your journey.
Siem Reap would be next, the waterbird sanctuary of Prek Toel is a worthwhile excursion, along with Ang Trapeang Thmor for Sarus Cranes.
Then relax and explore the wonderful Angkor Wat.
Enjoy the country, some fantastic birding to be had.
James