To Improve SQL performance, why not just put lots of RAM rather than having faster hard disks?

view full story

http://serverfault.com – People keep telling me that in order to improve SQL server performance, buy the fastest hard disks possible with Raid 5 and etc. So I was thinking, instead of spending all the money for Raid 5 and super-duper fast hard disks (which isn't cheap by the way), why not just get tonnes of RAM? We know that SQL server loads the database into memory. Memory is wayyyy faster than any hard disks. Why not stuff in like 100GB of RAM on a server?? Then just use a regular SCSI hard disk with raid 1. Wouldn't that be a lot cheaper and faster? (HowTos)