How do you size a server for ecommerce.  We have an application that
we are going to launch as an asp.  It has a web front end and an SQL
(mysql) back end.  Based on a growing number of users and transactions
how do we determine what our initial and future server requirements are
(e.g. processor speed, number of processors, amount of ram as well as
other more esoteric things like caches)?  At what point would we
expect mysql to "run out of gas"?

Sql databases, require very little amounts of storage space, our
servers run a 7000 product portfolio, storing transaction details for
over 1000 transactions and customers. Combining this with over 96000
data entries of various forms we have a considerable size database
which occupies less than 12mb of data.

Most servers nowaday have 20gb so you won't be running out of space
for a few years at that rate, processor speed and ram is only required
if your web applications use complicate procedures to display products
and calculations on the fly as it were, simple, add a box to cart
products could be powered on even the worst of machines.

Currently our site,s some of them very large ecommerce sites never
occupy more than 250mb of size, allowing plenty of room for play. This
is of course subject to people uploading sensible edited images for
use and not 1.4mb home digicam pics for the fun of it.

If you require any more help email me 
matt yabsley