caching & proxy server

i was thinking why do we spend hundreds of bucks on fast scsi raids for internet proxy & cache servers when you can get 4GB of RAM..create a 3.5GB RAM Disk with mirrored parition on hard disk and store the cache directory on this ram disk..should give hell lotta performance under load and im sure i can buy 4GB memory for the price of one SCSI disk..web cache servers dont need redundancy or anything so it can be fine even with a non ECC ram if you want to go even more cheaper