number of ecommerce sites

I realize this is a very open ended question and for that I apologize. I am trying to spec out needed hardware for an estimated 600 or so ecommerce sites using Virtualmin and a combination of oscommerce, zencart, magento,… (I.E. scripted installs from Virtualmin UI) that I can expect to run on a single server. Let’s assume a 3ghz CPU with 2048meg RAM with Virtualmin installed on CentOS 6.3. I am estimating about 100 to 200 Meg for each stores DB so HDD size is easy these days. These will be small business stores averaging maybe 150+/- sales per month. Any real world metrics would be most appreciated.

Thanks in advance.

I wouldnt worry too much about the speed of the CPU, I would rather look at the cache of that CPU. More is better, think 12MB cache.
2GB RAM is hardly enough. Think 16GB RAM or more if you want to host 600 eCommerce sites.
HDD size is easy to come by too, but it is a server so the HDD should be of some quality.
You need to think redundant, meaning raid 5 minimum. Now you need 3x 1.5 TB drives …

As ecommerce sites need to be up always, you can not afford to go down once. Not even to reboot after a kernel update.
Now you need to create a cluster, preferably in some elastic and scalable cloud environment.

You are expecting monthly sales of $90.000 (600x150) so such cloud environment would set you back about $1500 a month, but you would normally base 10% o your gross sales for operational costs.

In the real world one would not use 1 server for such a project, one would use several servers.

Thank you for your reply and yes, I do have load balance in place and redundant fast drives drives. My thinking for this, which I guess I could have better explained, is if I have load balancing and a server fails, or when I need to do kernel upgrades, or… What do I need to keep the sites from slowing unacceptably when on only a single instance. This will be a XEN environment with Multiple Dom0’s so every site will have a mirror on different physical hardware. So I was thinking in terms of what do I need for each of the Physical boxes to be able to host how many DomU’s each running a copy of Virtualmin. I tried to simplify the question but I guess I went to far. So based on what you have seen “in the real world” How many ecommerce sites would you expect to run well under these conditions assuming I assign hardware to each DOMU as above? Does that make more sense? Just trying to get some metrics to spec hardware on so I can estimate budget.