I've not done this run by taxi and I'm not sure if you're able to get one car all the way through or need to get two cars (one from Ho Chi Minh City to the border and a second from the border to Phnom Penh).
If it is possible I'd warrant a guess any travel agent in Saigon should be able to help you out.
#2 somtam2000 has been a member since 21/1/2004. Location: Indonesia. Posts: 7,800
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Drivers crossing borders run into major hassles - unless they work for a bus line or trucking company.
SaiGon Transit on Nguyen Hue (HCMC) has a daily run both ways in new international style buses, USD$12.
You can get your Cambodian visa on-line or at their consulate avoiding 'surcharges' at the border. And don't let anyone handle your luggage at the border, either.
#3 CatBa has been a member since 5/3/2007. Posts: 349
That's a really short bus trip--not 8 hrs. Other option is Mekong river overnighter cruise. Taxis costs lots and end up as way more trouble than it's worth. Take the bus--easy trip--4 hrs plus border time +1 hr. $20 USD for Cambodian visa at border--they will try to rip you off (surcharge).
#4 jimsparadise has been a member since 25/11/2006. Posts: 30