Rsawtt,
You have several option when going overland between HCMC and Phnom Penh. You can go by bus or a combination of bus/boat. One traveler wrote about his trip from HCMC to PP here. I wrote about my bus trip in my blog.
You should see the Cu Chi tunnels while in HCMC, also spend a few days in the Mekong delta. Another option would be to spend 2-3 days in DaLat. Or do it all. Take a city tour, a tour that includes Cu Chi, then spend 2 days in the delta. Take a bus up to DaLat and get out of the heat and the hectic pace of HCMC. I have plenty of photos of all this in my gallery. Send me a PM if you have questions.
[ Edit: Edited on Oct 22, 2008, at 11:43 PM by vegasmike6 ]