Things to consider:
Are you going to be racking the servers?
Are you going to by a model that will allow you to maximize internal expansion or go with a JBOD?
My reccomendation is Compaq Proliant but you seem to be locked in by contract to go Dell. If this is the case then go with the 2600 tower ro 2650 rack. I noticed you plan on growing in the future. If so look at the controller card and go for the higher cache amount for around 128. Do you know what kind of IO your going to have?
On the Proc I would go with two get the 1mb models.
Are you goinng to run Server 2003? or just 2000? If you can set up load ballancing I would.
Either way I would use the SCSI hard drive 10k's
As far as servers are concerned SATA has not proven themselves as longterm reliable.
Also what are you doing for email? Are you going to run Exchange?
The web server is that static or active content?