You shouldn't need anything too powerful I'd say. Navigating the viewport is fast on pretty much most systems really. There are plenty of tools built into both to help them cope with large data sets, such as proxy models, particles and such and most things have toggleable quality settings to ensure things can be done in the viewport speedily.
I'd recommend 6gig of ram minimum really so you can have other things running at the same time without worrying about it. The more the merrier really. Rendering is of course the biggest resource hog, and in that case its down to CPU and possibly ram if your doing something particularly detailed. GPU power is becoming more important now, as both have viewport options that utilise your GPU for much faster viewport navigation that support using more textures and such in realtime. Also, the fastest renderers utilise the GPU. I believe 3ds max Iray renderer uses your GPU entirely and I hear it's very quick but not so feature rich. If you wanted to go particularly high end, vray would be the best option for rendering, but it's made by someone else and has a pretty steep cost.