The performance improvement you will see depends on how IO bound your application is.
For SQL Server, you should look ar adding memory first to get performance improvements if you see long disk read queue lengths.
You can look at the links below for info on high end systems designed for situations where high performance IO is critical.
Fusion-io ioDrive
http://www.tgdaily.com/content/view/34065/135/
Intel’s X25-E Versus The Fusion-io ioDrive
http://www.tomshardware.com/picturestory/493-8-x25-e-fusion-io-iodrive.html
HP StorageWorks 80GB IO Accelerator for $4,400 and 320GB IO Accelerator for $13,200
The intel X25-E in a RAID array compared against SAS disks.
SQLIO performance:
http://it.anandtech.com/IT/showdoc.aspx?i=3532&p=7
MySQL DB setup with different array compositions: