Indexes are useful for finding existing data, such as in a SELECT, UPDATE or DELETE statement.The statement has to reference the column(s) in the index. Clustered indexes are especially good for finding a range of values.Personally, I want every table to have a clustered key. Without one, your data is "randomly" placed in storage (disk). The implication for updating a range of values is that, potentially, you'll need a separate disk read for every record of data. That's not good. With a clustered index, once you have found one record, the "next" one in the range is in the same location so you cut down on disk activity; always a good idea.
Long answer, short - a clustered index should improve your update performance.
Data inserts, however, can be slowed by an index since there is some overhead in maintaining the index as well as the data. How much you'll need to test. Usually the overhead on the inserts is outweighed by the improvements in the selects, updates and deletes. In a worst case scenario, you can drop the index, insert your data and re-create the index and that can take less time than simply inserting into an indexed table. Will this be true for your environment, only testing will tell. Depending on when and how you insert the data (off hours?), you may not care if the data load takes slightly longer to complete.