I have gone through a question within the forums asking "smaller disks or more spindles" which is better for performance. The simple answer is more disks to handle the data growth and also mostly depends upon the number of transactons that are handled with a sensible normalization, as the transactions method & the CHECKPOINT method are important to assess such information. Further if that database is currently in live and if you are looking to upgrade the hardware then the best option is to go...(
read more)
Read the complete post at http://sqlserver-qa.net/blogs/perftune/archive/2007/08/10/1347.aspx