If you want a system for real growth go for a SUN 64bit UltraSPARC III processor. You can start with a single processor and scale to 24 in a single box, then cluster for more power.
PC processors and Windows just do not scale. You are probably after just a small server systems but there are so many stories of startups setting up servers on windows and then choking when they started to grow.
In a few years, Itanium/Hammer & Linux should be a good solution (especially with the investment IBM is making into this area of Linux development).
I don't think Windows will ever cut it, W2K Datacenter has been a bit of a failure and just highlights the fact that you can't make a personal OS be a highend server. Just look at what Hotmail is running on!
<font color=blue> The Revolution starts here... as soon as I finish my coffee </font color=blue>