Dodger is correct regarding driving to Siem Reap from Buriram. There is no highway to SR from Buriram . . . but there IS a major highway (Cambodian National Highway #68) from the next province Surin (Kap Choen District), which from Buriram is only a half hour to one hour drive depending where in Buriram you're journey starts from.
You will have to pay approx 4000 baht 'tax' for the vehicle (whether owned or rented) at Cambodian Immigration as well as any tourist visa costs and then you are on you're way, no problem.

The obfuscaters in this thread (Pong being the worst example) do not know what they are taking about.