I think you are overestimating ITOps10. This is ok but you will spend a lot more then you need.
Another way to approach it is to buy a nice midrange machine and go live. See how it works. You cna always add another, tougher machine as a clustered solution and this way you are not buying hardware you do not require.
My experience is that people tend to dramatically over state their requirements.
For example, if you are just displaying TIFF data then the size of your server is not critical since its a static file. If you are dynamically GENERATING TIFF data on the fly that is much more intensive.
Your looking for a simple answer to a complex question so my advice is 'go midrange with a 3GHz, 2GB RAM P4 system and grow into it.'