I am just working from memory, but I think you only need the slim services card if you expand to 8 x 24. Could be wrong, but those slim cards are expensive and if you do not need one, it is expensive shelf stock.
To go 8 x 16, you would need a 2nd trunk card (to go from 4 to 8 incoming lines, get a CI card if you want caller ID), and then as curlycord said, new non-restricted software card. As for the level of that card, 7.1 is well tested and stable. Unless you plan to buy some upgrade software tools, you will probably end up reprogramming as curlycord indicated, but with a 4x8, you really do not have much programming time in that chassis anyway.