Windows 2003 or Windows 2008 based on my server specs & needs...

Hello,
I have a network of 13 workstation clients (WinXP SP-2 PRO) and a server with the following specs:
Intel Pentium 4 640 (Prescott Hyper-Threading) 3.2GHZ FSB800MHZ 2MB L2 Cache (Not multi-core CPU!!)
DDR 2GB 400MHZ Dual Channel (PC3200 200MHZ)
300GB HD
ATI Radeon X330/X550/X1050

I want to setup a Domain and so I have to choose from the following two operating systems to install in this server:
Windows 2003 Standard R2 DSP License
Windows 2008 Standard Sngl OLP NL GOVT (Government License)

From my previous experiences, I know that Windows 2003 will work perfectly on the above setup:

Windows 2003 Requirements
http://technet.microsoft.com/en-us [...] 30827.aspx

But the Windows 2008 government license is far better than the Windows 2003 DSP which can only be used for one PC/Server. (It gets tied to the motherboard & CPU, where as government license can be carried to the next server in case I buy a new one in the future)

I checked the Windows 2008 system requirements and it should work fine on my server:
Windows 2008 Requirements
http://msdn.microsoft.com/en-us/wi [...] 96364.aspx

But I know that these requirements by Microsoft are never accurate and since I have never actually seen it in action I can’t judge how well it would work on my specific server. (My CPU is not Multi-core)
The services I’m interested in installing are: Active Directory / Domain Controller (With 13 Domain users), DNS service, DHCP service (The Server will give Internet to the Domain PCs via the DSL/Router) & File Sharing service. (Possibly even setting up a VPN tunnel via the DSL / Router in the future to connect to another workgroup via the Internet)

So based on my setup and needs, how well would I expect Windows 2008 Standard to work on my server? I would rather avoid upgrading the hardware at the moment, but I don’t want a slow/jerky server & Domain either!
Should I get Windows 2003 Standard R2 instead?

Any suggestions will be highly appreciated.
Thanks in advance.