Agree with Than staying right in Kruger park and self driving is an amazing experience and the two camps he has suggested are in fantastic game rich locations. This involves self driving and is what thousand of people do every year in the park. Generally you will find agents do not offer this option as you are staying in a national Park, therefore they are more likely to recommend the more expensive private lodges.
So it seems you may have already decided but if not your first decision is whether you are wanting to stay in the park and self drive or stay in a private lodge where everything is included but there is a huge difference in price. Some of the best private lodges are located in the the greater Kruger adjacent to the main park as Jacqui has suggested .
Both are wonderful but completely different experiences, I won't go into more detail yet not knowing if you have made that decision.
Depending of how much time you have, a combination of self drive staying in park accomodation with a few days in a private lodge can give you the ultimate safari. One way you get to find and enjoy wildlife from the comfort of you own car and can game drive for twelve hours a day if you like, with all the flexibility you want. The other way is generally pretty luxurious and will include two game drives a day of around three hours in a safari jeep that will also be able to take you off road for a great sighting.
As for how long this is really up to the individual and how much you want to see and how much you love wildlife, very much a personal preference. We usually do a combination of both, but I think you would need at least a week to get the full benefit of that.
For some three or four days may be enough, for us three weeks is not long enough !
Edited: 9:00 pm, May 08, 2013