multiple data file in user database

Hi All,

I have three database on same server with three drive F,G,H

A databsae size is 400 GB.

B databsae  size is 60 GB and C is 800 MB

and B db 50 % select and 50 % insert transaction

B database has mdf and LDF in same disk (H) with Raid 5 and SAN

A and C database has mdf in G with Raid 5 and SAN and LDF in F for both DB with only Raid 5

A DB 400 GB space is finished in this db insert is more than select

now i want to think add ndf file in A DB but in what drive i  am confused ...

If I add ndf file then how its work when i transaction is occur or index  rebuild time

Is it right adding ndf file or add space existing mdf in tems of performance

Thanks