I'd say age is the biggest factor here. The fact you're even thinking about not buying the current version of the operating system (seemingly without good reason) is crazy to me, but that's my opinion I guess. Also, when you say Windows Server 2008, I hope you actually mean Windows Server 2008 R2.
More importantly however, the final set of security updates for Windows Server 2003 will be released on July 14th 2015. That's only 4 years away and will come round so much quicker than you think. Unless you can be absolutely certain the server will be replaced before the operating system reaches end of life, you'll be running production (and I'm guessing business critical) applications on an unsupported and vulnerable operating system before you know it.
Not only is Microsoft going to shun you when something breaks, so will your application vendors. If you have any kind of problem, you are pretty much up the proverbial creek without a paddle.
Then you've got your security problems. There WILL be vulnerabilities, and Microsoft WILL NOT patch them. Try explaining to your boss that your server is down because it's been hacked. Then try to explain that when you re-install Windows, the attackers can do it all over again because there is not and will never be a patch to fix the underlying security problem.