IO Bottleneck on SQL Server 2008 r2

Using dynamic views, I found that one of the data files' read performance is > 100ms.

ie..

SELECT mf.Name
FROM sys.dm_io_virtual_file_stats(NULL, NULL) AS fs
INNER JOIN sys.master_files AS mf ON fs.database_id = mf.database_id
AND fs.[file_id] = mf.[file_id]
WHERE ( io_stall_read_ms / ( 1.0 + num_of_reads ) ) > 100 ;

The storage subsystem is SAS 146 GB 15K 6Gbps disks.  2 Disks mirrored are used for os.  4 Disks are single drive raid 10.  This is where I have my dbs are located. 

I am suspecting the sql server may attempt to write too often.

Can I control how often sql server writes db/log files to disk.  If yes, how?  

Will splitting the files into data and index files help?   

Any other suggestion is appreciated.

Thanks in advance