Web servers - cheap and many or expensive and few?

That's a tough one.

Google proven that you CAN use commodity hardware, but you'll need some good programming practices so that your software will be able to accept the fact that failures will occur. This means you'll need to be able to expand to multiple parallel servers to avoid extreme cases when the hardware will fail (and it will).

Of course, it also depends on the budget. Just think how much money Google would have to spend to buy 100,000 high end servers instead of commodity hardware...

Even the most expensive hardware is going to fail. Hard drives come to a stand still. CPU's overheat. A trillion things can go wrong.

If you have a small web-site (or a few small sites), just put yourself together a basic server and run with it.  Larger sites require more planning.

 I researched this a lot, and have come to the conclusion that it is far cheaper to create a redundant cluster than spending the money on expensive servers. I am an Apache/MySQL/PHP fan and firmly believe that creating a cluster is the way to go (especially if you are using linux) when using these tools. 

To build a cluster you probably want 5 cheap servers (<$1000). Note that $1000 can buy you a REALLY fast server (multi core; raid; gigabit ethernet). This is roughly $5000 in components for 5. Most high end servers cost $2500. So for the same price you can buy two servers.

 Don't forget to include backup batteries when you are estimating the cost. My guess is that your biggest cost (time and/or money) is going to be setting up and maintaining the servers - whichever route you take; oh yeah, and bandwidth.  

First of all...in order to get a good answer for this question you need to supply more information.  Have you made a scalability plan for this site?  what type of application are you planning to run? Do you have a budget?

Both venues have their pros and cons, if you a large quantity of cheap machines you will need more robust load balancing and networking facilities.  If you higher performance machines you may be able to reduce the cost of this infrastructure.  Take into account the type of app that you plan to run and the type of related infrastructure such as DB servers and such. 

In addition, if you settle on a brand name you can create standard images that will last for a while (18-24 months) which means that adding the next server will be a very simplified job as opposed to installing each separate machine with its relevant drivers and different HW configurations.

If you post more information, I will be able to give you more accurate advice.