hmm, normally, I'd say you could get away with dual workstations running hearbeat/mon software....but since you're running windows....
It is my experience that unless you have a way of providing some sort of fault tolerance(heartbeat,HA Linux, LVS,BSD Cluster,RANT), you will probably want to go with one of those "laughable" pre-builts from dell or gateway. This is not to say a commodity PC built from off the shelf parts wouldn't serve, it's more for that one or two percent more stability and fault tolerance and testing that goes into a server system as I've always lived by 5 words when it comes to setting up servers, downtime is not an option. Granted, it happens anyways, but you want to try and avoid it. I have always recommended netfinity servers myself as I have always had good experiences with them but they are costly. You could try tigerdirect for a server or get an array of redundant nt systems together, tho you won't have a whole lot of horsepower.
I would just keep in mind that it doesn't sound like you need a whole lot of horsepower for your needs. Don't go for the dual p3s with a gig of ram, because small ecommerce sites will probably not use it, even if they are spiking. a mid-high end single cpu p2 will probably serve your purpose easily given 256-512 MB of RAM. And these are conservative estimates...you could most likely get away with less. The general idea is to lower cost by saving on hardware as well as brand.
If you do end up building your own, I recommend an all intel system. Yes, it costs more, but a board from intel is generally more stable than a board from a 3rd party manufacturing for amd. Granted, this is a minor point as AT is hosted on AMD machines and is plenty stable, but it's just my observations. I'm sure someone will come up with AMD is plenty stable arguments and anti-intel arguments, so I ask you take this suggestion with my opinion in mind.