You can get an idea of suitable systems by speccing one up at Dell.com for example - gives you a starting point at least. Even if you can't specifically quote prices/specifications from a site.
Office work would be a low spec system, budget Sempron with 512mb ram, 80gb HD for example, laser printer - , Web Development 1024mb, decent size HD (160gb+), games development, loads of memory, big HD, good 3d graphics and perhaps dual monitors.The latter could apply to the web development PC too.
Take a look at some specialist sites and see what systems they aim at particular kinds of users.click here for example
Just my 2 bob