Best Hardware for SQL Server

I am looking to purchase a box to run SQL Server 2000. It will host a fairly small DB but I would like some room to "grow" the system. So I am going to get a chassis that can hold a number of SCSI disks. My question is this: The box can accommodate up to dual Intel Xeon processors at up to 2.80GHz. Initially I will start with one processor. My alternative is to get a single P4 2.4GHz with the same disk.
How much "slower" is the single Xeon then the single P4?
How much will upgrading to dual Xeon's get me over a single P4?