I know the Dell T-710 is no spring chicken but it does have 8 cores, is it really that dated?
I use the Active Directory primarily for user access control and the server is also the centralized data depository for commonly used forms and the storage depot for all the old medical record that are scanned from multiple workstations to the same folder location on the server. It is also the usergate proxy and firewall for user internet access.
What is the best current, stable operating system that you might suggest to replace Server 2003? How much are we taking about? I'm curious about the swing migration that Bert mentioned. Is this an online service? How expensive it that? Does any one have personal experience with it? Bert did you use this make an OS or server tranisition?
Correct me if I am wrong, but you do not need a lot of processor power to run Active Directory, run the firewall and serve as a document server. From that standpoint, no upgrade would really be needed.
What you could do is to virtualize the current server and put it on another system that would be 64 bit and give a more memory to run AC and SQL server while still running Server 2003 to get the Services currently running. This Virtualization could be run on Windows 8 with hypervisor or even Windows 7 with a third party virtualization software.
Another option would be to set up another box with Win 7/8 and 8-16 G of memory to run AC/SQL and leave Server 2003 alone to do it's thing.
Your server hardware is capable of handling much more than what it is doing. Your primary limitation is RAM and is because of Server 2003's 32 bit status. Other than memory, there is no good reason to move to another system.
SBS 2011 Essentials is much easier to manage, would go backups easier than your current system and could be clean start to begin again. 2012 Essentials has just been released and is not the same product, it is closer to what was SBS regular minus exchange and sharepoint. Not sure about price but I think they are about the same. 2012 may work with Windows 8 better but has JUST been released. Both are still based on System 2008R2 core as Sandeep pointed out above.
If it ain't broke... More memory may speed things up, but probably more in relation to AC. Do you have a gigabit network. That would probably make a bigger difference than messing with the server.