This has changed a little bit in the past few weeks.
To get to the latest 5.0 version:
1) Install the 5.0(1) major update.
2) Install the 5.0(6) service pack
3) Install the S222 signature update (or the latest sig update)
To get to the latest 5.1 version:
1) Install the 5.0(1) major update.
2) Install the 5.1(1d) minor pack
3) Install the S222 signature update (or the latest sig update)