Yes, it's a very easy drive now (3 hrs one way from S.R.). I've been twice (feb 2011 and feb 2012). Any agency downtown S.R. can organize this for you, but prices can be very different. What I did the 2 years is "renting the car with driver" for the whole day. You leave at 6:00 am from your hotel in S.R., arrive 9:00 or 9:30 in Preah Vihear, spend 3 hrs there, and go back to S.R. via Koh Ker that is very interesting to visit. The road from P.V. To Koh Ker is brand new as well, so no pb. You will be back in S.R. around 6:00 pm and you'll have a great day.
For this tour, I found the best price (100 USD) at the following agency :
Two season travel tour. They are located downtown S.R. near the old market. Entrance is on street 9, close to corner with street 10. They have been very reliable the 2 times I used their services.
What you can do is when you arrive in S.R., go to their offices and arrange this for the day you wish. No need to book in advance, as you just need a car with a driver and they have plenty. Of course, keep in mind that a driver is not a guide. You can ask for a guide but of course, it will cost you more.
Enjoy this wonderful temple while it is still not part of the group tours... it won't be long before they invade this place