There are a lot of questions you have to ask yourself, to decide which technology to use.
Your first decision that you need to think about is your operating system. There are two options, Windows vs. Linux. Which is better? In my opinion the question should be, which is better for you.
Do you have an existing software infrastructure? If so, you'll need to stick with the OS that supports your software (ie: asp = windows, php = linux) though you can run both interchangeably, I wouldn't reccomend it for production use. If you have an asp infrastructure stick with Windows/IIS. If you have php, stick with Linux.
So lets say you have neither? Here are some points to consider:
(the following are merely opinions of mine and do not reflect the opinions of my employer)
Are you a small company? If so, you may want to consider using Linux, due to the fact they have more open source programs available, and labor is generally far cheaper for a php developer than an asp/c#/.net developer. While there are open source packages available for .net software, they are few and far between.
Are you a larger company? If so, you may want to leverage the extra capital available to get enterprise software with professional support. I am not saying Linux does not offer quality software, but as a general rule, Open source software is not always supported, and with .net software vendors what you are paying for is support, and a guarantee of functionality. If you do not have the time to "roll up your sleeves and dig in" to your software or architecture, it's nice to pay someone to do it for you.
So that's your first big decision. There are tons of other factors as well, once you decide on a platform. If you would like to discuss this further, please post any questions you might have, and I'll be glad to try and answer them.
Typically, a 2 socket server (Xeon 5000 sequece) is chosen for web applications and infrastructure for most medium to large businesses environments
If you are a small business and your web serving needs are moderate and you don't expect or need a lot of growth built into your server for the next few years, you may only need a 1 socket server. Like the response from Jeremy, having some more information would help a lot. In the mean time, here is a server selection guide that will give you an overveiw from a hardware perspective.
Also you can read more about the various products that intel has at www.intel.com/go/xeon