Ok. We know database size, number of concurrent users, number of transactions per minute; should choose number of processors, RAID, RAM, mirroring and clustering.
There are no exact rule.. but may be there are no rules at all?
In my practice in every case I have "legacy" system, and after some inspections and interview I can form an opinion how hardware and design can be improved.
But every time when I meet "absolutely" new system (I guess there are no new systems, but sometimes are such tasks) I can't say anything trustful. So I'm interesting how people deal with such tasks? They map task on theirs experience or have some base formulas?