You can't buy 1 windows license and install it on 100 systems (for example).
It's mostly 1 license per system.
If you use it for hosting purposes, go for an SPLA license with a dedicated hosting provider if you also take a server there.
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If you can wait for one month, windows 2008 R2 was just RTM'd and will be publicly avaiable in August. And Microsoft is trying very hard to price it right to attract 1Socket to 2S web tier servers to switch to Windows Server OS, so I expect them to be more aggressive with R2 than 2003 or 2008R1.
If you can't wait for one month, then license windows 2003 for web tier and license 2008R1 for Database tier.
I would strongly recommend you install 64bit Windows 2008 Core. You get all of the services minus the GUI overhead. The performance increases with the disk I/O, self-healing NTFS and address space allocation for system resources alone are worth ditching 2003. IMHO