We're building a new network with 5 locations and 200 users and would like to know if we should be building it on 2008 or 2003 servers.
Newer is typically better (when its not bleeding edge)
Windows 2008 is FIVE YEARS (in Microsoft time anyway) newer than 2003.
Quite a bit happened in those 5 years...
Unless you have a GOOD reason to go with 2003, I'd pick the newer model. ;-)
Unless you have a legacy application that doesn't support 2008, I don't see any reason to deploy a brand new netwrok with 2003.
IMO 2008 has proven itself as worthy a replacement to 2003.
Go with Windows Server 2008 R2. Thats your best bet to support the new Windows 7.
With 2008, you also get IIS7 and better support for advanced WCF features, if you decide to expand into WCF web services. I don't see a reason to not go for 2008 if you can.
2003 is a 6 year old platform. Put it in now and you'll likely be upgrading to 2008 in a year's time.
Make sure you take into account the eventual need for x64. Exchange 2010 and later will only be available on x64, and Windows 2008 R2 only comes in x64, while 2008 comes in both x86 and x64.
As the other guys say go 2K8 unless you have a specific need for '03. That said now sounds like the ideal time to consider whether you really NEED Windows at all, it may well be the perfect OS for you but it might be worth doing a Pro/Con list at least vs. Linux or OSX...
Consider that a Systemstate Backup of a 2008 domain controller takes about 8,5 GB now. Don't know about other Roles...
But go for 2008, don't set up deprecated Software...
Don't expect to run Quickbooks Enterprise Qbi series via Quickbooks Database Manager on Windows Server 2008 if you have servers in Australia as the Australian version (only) doesn't support Windows Server 2008. See http://home.quicken.com.au/Pages/ProductDetails.aspx?pcode=27&pcatid=12
Very poor form Intuit / Quicken / Reckon.