The one thing I would like to add is for it to last you 3 years, you may want to invest a little more now to get something with a dedicated video card, and 64 bit support.
With Vista coming out, and now that Intel has 64 bit capable machines, I think you are going to see a real push to 64 bit apps. 3 years is a long time in the computer world, and I would hate for you to save a couple bucks now and find you can't stretch the product life cycle another year (or more) making your cost of ownership that much higher.
$1,500 / 2 years of service = $750 per year.
$1,800 / 3 years of service = $600 per year.
$2,000 / 4 years of service = $500 per year.
$300 invested upfront actually saves you a considerable amount of money and a lot of hassles in the long run, and gets you a machine you could really love.
As far as years of service, if the machine is going to be used for everyday things, working with office apps, a little gaming, etc. I don't see why a current system can't last 4 years w/o a problem.
I mean, I am using a Zatoichi right now, and running a couple MS Office apps, and web browsing with a 3 windows open. I have dual 2 Ghz CPU cores at my disposal, but am using 800 Mhz of processing power. The CPU is like having Ronnie Coleman standing in my kitchen incase I need to have someone pour a gallon of milk for me.
Windows Vista Aero Ultimate needs a DX9 capable card, no problem, you can run that w/o any problems whatsoever, ultimately... what more does the average person need? The only thing I can think of is a quality machine with high quality components.
I think people desire to have more power and that will always continue to go up, but the actual need for more power just isn't there for 98% of the people out there because these computer systems right now are so stinking powerful.