Server Sizing

Couple things you need to consider. One is how much disk space are you going to need? Also how frequently are your users going to be acccessing the data or writting to the disk simultaneously? If its a lot then you'll want to get scsi drives, for heavy use and/or frequent concurrent connections you will really see the difference. Sata drives are cheaper and thus are a bit more appealing, but having worked with several servers with Sata drives I absolutely hate them (for servers). Compared to scsi their at least 33% or more slower than scsi, and don't handly concurrent conections as smoothly as scsi. I would only consider using Sata drives if you need a very large amount of storage (like a TB) and generally only 1 person will be accessing the disks at a time.

CPU speed is also heavily correlated to what applications you will be running. If you're going have a large sql database or other large application you'll benefit from multiple cpu's or cpu's with multiple cores. If the server is going to just be general use and not have any intensive apps run from it then getting a single fast cpu should work just fine. Dual core cpu's are great, but you need to have apps that are coded to make use of multiple cpu's, otherwise the 2nd core will sit idle most of the time and be a money sink.

a couple of extra points: 

scsi drives are expensive and require extra hardware -- consider a pair of raptors as a mid-way point if you can't justify scsi (it's what I'm doing with my server).  they're almost if not equally as reliable and they'll be good performers for a server in raid 0 (but still make nightly backups!).

go for dual core -- it doesn't cost *that* much more at this point and is a good balance between single core chips and say dual xeons (you question would indicate that you don't really need dual xeons ;p).

following from the above, keep in mind that programs like SQLServer if you need it do cost more for multiple CPUs

lastly, depending on your server uses, you may find winXP will work just as well for you as Server2003, at a fraction of the cost.

Take the "Required Resources" from all Applications that will be running "AT THE SAME TIME" , add them together count the other programs that will be loaded and 10% of the previous total for each one take that number and Double it.  Toss in 80GB of random storage for yourself.  If you are running things that will have Customer/Sales/Inventory Databases that tend to get really big into the terabyte Range look at Network Attached Storage RAIDS and other such monsters.

I never have had Processor or memory Issues using this "Rule Of Thumb" and no matter how much HD space you put into the system someone or some group will find a way to use all of it so leave room for expansion...as a matter of fact try and design the entire network to grow and expand as your company grows and expands.

Also if you're going to need something like SQL server; which combined with the cost of 2003 server is going to be very expensive you might want to look at using 2003 SBS Premium Edition. It includes 2003 server, Exchange 2003, SQL 2000 and ISA 2004. As a software suite its a great value for what you get, far cheaper than if you purchased all these programs seperately.

If Dell offers the Raptor drives those would be pretty much the only drives that can come close to matching scsi performance at a cheaper cost.

Right On,
Might want to consider Colocating your server in a data center near your office to ensure bandwidth and uptime. you an usaly find some body to do it for under $100.00 per month.