Not something to worry about mcuh. Just put everything in the clustered key and all is well.
create table tbl_statistics
(p_key int identity(1,1),
client_id int not null,
client_datetime datetime not null,
constraint CI_rowId_ClientId primary key clustered (p_key,client_id,client_datetime)
If you'd like to have datetime as nullable, than removing client_datetime from the index and it will not cause any changes . The table has just three short columns.
SQL Server developer at Seavus