You would probably be set with a 3.2Ghz P4... There's little point in going Xeon, because it is dual-CPU capable and you wouldn't use that. I've seen some benchmarks and, if I'm not mistaken, Xeon doesn't offer that big a difference in such fields. You might try Xeon, though, if you want more memory... some Xeon mobos offer more than 4GB support (usually dual-cpu mobos...)
And on a good mobo (maybe even Canterwood), the memory performance of such a machine would be very good. So you'd probably be set with a high-performance HD array (RAID or something), a 3.2Ghz P4 on, say, P4C800 Deluxe, and 4GB of DDR400.
As I understand, Opteron also does a reasonably good job at SQL and such. It might be an interesting alternative, but bear in mind that Opteron is, at this point, brand new... I'd stick with the P4 3.2Ghz for now (which should give you respectable performance). It's just my conservative side here... But I'm just saying, hey, you could try Opteron if you wanted. It will also be more expensive, that's for sure.
On windows 2003 sure a P4 will offer good peformance and canterwood support ECC and CSA can be use.A onboards native SATA can be use unless you want to go at a high performing SCSI level if so a Xeon become more useful as they feature a PCI 64bit or better but CPU and mothers boards come at a premium price compare to normal P4 and will be slower.Get sure that you will need this power as aceshardware website run on a P3 500 mghz and is able to give 1 million transaction a day.
But do you think MS has already optimized Windows 2003 and SQL Server for the Opteron???
I'm not sure as to that...
If I were you, I'd go with the single P4 3.2Ghz on Canterwood. It's a very powerful combination, and it's based on tried-and-true technology. Opteron is new, and it's hard to really place a bet on such new technology. So go with Intel's 3.2Ghz... this is my opinion anyway.
...and for those who think that this has in any way to do with Intel vs AMD and me preferring Intel, I'll just say this: I said exactly the same thing when Itanium was launched...
...If you want to try Opteron, it's your choice, really.
No! Wait! I found an interesting <A HREF="http://www.aceshardware.com/read.jsp?id=55000260" target="_new">review</A> on Aceshardware that benchmarked performance specifically with MS SQL. And Opteron 1.8Ghz can't really outperform Xeon at all... this is a very good indication that, even with all its bells and whistles, Opteron will certainly not be better than the 3.2Ghz P4 with HT on Canterwood - which also has its bells and whistles regarding memory system, mind you.
So, it just became a hell of a lot easier to give you reasonable advice: go with the 3.2Ghz and you're set. Opteron is probably just a waste of money in your case.
The Xeon is pretty much a P4 with SMP capabilities. So when using one processor there won't be any difference between P4 and Xeon although one might expect the Xeon to be more stable in the long run than a P4 due to the higher selection criteria for SMP processors.
But your real choice of processor is going to be determined by the requirements you have for your server. How is it going to be used, how much will your company depend on it, how big will the databases be etc. etc. Based on these you'll need to decide if you need SCSI for fast I/Os, gigabit ethernet, fibre channel connectivity, high throughput RAID etc. All of these things are only really usefull with a PCI-X bus which you will only find on a server board. Most server boards are for Xeon processors, indeed, I don't think there are P4 boards with PCI-X support. The Serverworks chipset can support P4 or Xeon with 64bit/66MHz PCI slots which is an improvement but you'll still be limited to a 533MHz FSB. Depending on what OS you are running, which verson of SQL server and how big your databases are you might find a server board is better anyway due to much larger RAM support.