no need flash in most cases unless you taking self portraits or group photo and got no choice but is back lighted. pop up flash will suffice in most cases. (unless your lens so big that cause a shadow )
http://picasaweb.google.com/yanyewkay/ quite a number of the pics are taken while on a tuktuk.
give you an advise.. skip the angkor night market. if you MUST visit, ask the tuktuk or driver to wait outside for 5 mins while you go in to look around.
I went there with only a 17-55. more than enough, then again depends on what you shoot.
the night skies are really beautiful for star gazing. if you into astro, bring your binos.