When deciding to deploy a server I would think more about the software than the hardware (from my experience). Linux servers will run on basically any hardware platform with ease while WIndows Server operating systems require very beefed up servers.
I own and operate an IT consulting firm with a corporate server running Ubuntu 9.10 Server Edition. I built the server using old desktop parts which equated to an AMD Phenom 9950, 4GB of DDR2-1066 RAM, and 2 x 750GB Hard Drives in RAID 1. Seeing as how I had the parts already and the operating system was free, my server was completely free-of-charge.
The other big thing to worry about with your own server is bandwidth. Bandwidth, in my opinion, is just as important as the software and hardware you choose to run. Low bandwidth will kill your server operations and high bandwidth can come to be too expensive. When it costs too much you might think about getting a server hosted at a hosting firm like Rackspace or one of their competitors.