I have 10 years IT experience, the last 4 in designing and building websystems for IBM. Buy the most you can afford, the Xeon processors machines are your best bet, they can handle many more instructions than the normal pentium 4 processors and the faster cache and bus speed will make a huge difference. Also whatever you do DO NOT RUN WINDOWS on the server if you want the best performance, we have had the best luck running Debian linux on dual processor cheaper servers (dual 800mhz servers with 1 gig of ram in each and 2 18 gig scsi drives) setup in a lvs cluster (so you can pop in machines as your traffic grows)
All this software is free and availible on the web and with just alittle linux experience you can have a smoking system for very little money. ICQ me and i can explain it more to anyone interested