Ubuntu Servers :: Minimal Hardware - Business Creating Webpages

I am thinking of starting a business creating web pages and supplying a 'enterprise server' type solutions, whilst still being highly cost effective - eg recomend the client to use an 'old' pc for the majority of their server needs (LDAP, mail, firewall, web server etc). I plan on doing all of this on a linux platform, so as to pass the cost savings of related 'microsoft enterprise systems' to my clients ~ thereby making my proposition more interesting to clients. However I'm not sure if the 'old' pc idea will 'cut the mustard' in terms of serving web pages. So my real question is, at what point does the speed of the internet connection reach a bottle neck with the speed of my CPU? As an 'example' in case I'm not being very clear (which I'm not sure I am).

My old pc has an AMD athlon chip in it, and equally old 30GB and 40GB HDD (SCSI ~ did I mention that they are old!). My intention is to set up a system to the above type specification at home, to see how long it takes me to do, and to give me an idea of what I should charge clients. I understand all the technology, but initially (to save on personal startup costs) I was intending to use my old pc as my personal gateway to the world, $40 for the web registration for 1 year is a considerable saving compared to the $20 per month for a hosted service ~ although as soon as things are going and being 'profitable' I would most likely either upgrade my server or get a hosted service. Any tools I can download onto the server to 'determine' the power consumption over any given period?