RAID Recomendations

[Ayman Altounji]

I'm configuring a ML370G2 for a client. It will have Windows2000, as a DC,GC. It will house Exchange2000 and SQL2000.
I've put Dual PIII 1266mhz CPU's in it. 512MB PC133 RAM. All storage is internal and I've added a 5i Controller which will have 3 18GB 10Krpm Disks on it.
However when looking at server sizing my box only just makes it, it really drops on Disk Sizing recommendadtions. I wan't to put in a RAID 5 across the 3 disks, which will house all data, app's and OS. Compaq's sizing tool says I should have 3 seperate physical arrays. Please note these servers have no more than 40 Users on them !!!!
DOes anyone know if I'm doing the wrong thing ? and if so point me to any hard doco on why ?

Regards

Cory Grant
cory@push-f1.com