how to determine the hardware I need

Hi,

I need to determine the hardware I need to host a postgresql database.
The problem is, I have to make some guess on the number of tables,
records/table, etc as it is a new website that will be launched. Needs
will evolve, and hardware has to be prepared.
The other problem is I don't know how to evaluate hardware needs based
on assumptions about the number of tables, rows/tables, number of
queries/day, ....

I browsed the history of the ML, but I didn't see an answer to this
question: how does one evaluate the hardware needed?
>From my readings, I have this idea:

RAM is important, but how do I evaluate how much RAM I need?
Putting logs on a different disk than data is important.
RAID: doesn't give a performance boost, but increases reliability.
CPU power isn't that important.
SCSI can be more reliable in extreme circonstances, but IDE does a good
job also.

Remaining questions, knowing that Apache will run on the same computer:
-How do I evaluate how much RAMI need?
-idem CPU. is a dual CPU an advantage?
-how much space will the database take?

If you have good pointers to documentation, it would be very helpful
too!

Thanks for your help!

Raph