Mmmm....yeah that sounds do-able, as long as you don't mind being on the go. Honestly I'm not too sure about the Mekong Delta, we didn't have time to go there, so I never really researched it too much.
I wish I'd had the chance to get the boat from Ho Chi Minh to Phnom Penh, but needs must I guess! I definitely think that a day/two days is enough for Phnom Penh, there isn't that much to see, with the exception of the usual markets etc, and obviously museums relating the the Khmer Rouge era (which were very sad, but very enlightening), and the killing fields (I think about an hour out of the city), but I didn't feel the need to go there.
Even if you do take a bus from Phnom Penh to Siam Reap, its not too long a trip, I think only about 4-5hours (maybe even less). How long are you planning on staying in Siam Reap? We did 4 days, but I guess it all depends on how many of the temples you want to see (there are many, with some about an hours tuk tuk drive out of the main area, but we found these ones very enjoyable, and very different from the central (very touristy!) ones, and far fewer visitors venture to these!) Don't know if its something you're interested in, but they do some fantastic cooking courses in Siam Reap, mainly half days, but relatively cheap and really good fun, and the food you cook is delicious!
The only other area in Cambodia that you might want to visit is Sihanoukville (well the islands off the coast really!), but that is basically only relaxing on the beach.
Hope some of this information helps....!