Custom fields shown dinamically in UI

Hello experts!

We are implementing SAP CRM 7.0 and we have the following requirement:

According to the value of a field that is selected (Drop-down list box) the rest of the fields vary according to the value selected. We are using an Activity object to model the requirement. There are many fields (85% custom and 15% standard) that depend on the value selected in the drop-down list box, aproximatelly 65 for each option and they are different data types.

For example:

Drop-down: if we select Option/value "A", then the fields that should appear below are

Field 1

Field 2

Field 3

...

...

Field 65

Drop-down: if we select Option/value "B", then the fields that should appear below are

Field 66

Field 67

Field 68

...

...

Field 130

Many thanks in advance,

Pablo