don't you think 5 years is a long time to keep a computer? if you're going to keep it longer than that then you should go with a powermac.
i bought my powerbook 1 yr ago, and i wanted to keep it 5-6 years. i'm running tiger on it now, and it should run leopard just fine...so that will probably take me to the 5 year mark, especially for the stuff i do.
the question you need to ask yourself is what kind of programs do you run now, and will run within the next two years (not what you would like to run, but what you will run). with that info it's much easier to make a decision and be content with it because your computer will do what you need it to do.
also keep in mind that with a fast hd (7200+) and tons of ram (2gb+) os x runs hella good.
you can upgrade the hd in the imac as far as i know, maybe get a 10k drive. defenitely go with the best video card you can get.
as far as building your own mac...that's not possible right now.