You can run DHCP (and it's integration with DNS and Active Directory) without doing the routing on it. Just disable it on the central router and let the w2k3 box handle it. You may need to set up DHCP forwarding on the switch (but not if it's all on one subnet already).
The advantage to a standalone device like a router is reliability. A box with no moving parts and not running anything else is less likely to give you trouble over time. No software updates, no overloading based on other tasks, etc. I'd be pretty hesitant to replace a standalone router with a computer, regardless of OS being used. Computers generally consume more power too.
As for bandwidth control you might want to start with monitoring first. Start tracking which ports are using how much bandwidth. Tie that into reports linked to DHCP lease reservations. You can often better manage network consumption based on reporting. As in, show who's the culprit and let management view the reports. Let the HR policies slap them around for wasting company bandwidth. Keep the IT department out of the enforcement loop. Just report the facts. Try this first, it's often usually more than enough. Going to network shaping and bandwidth throttling is often a lot more work to setup, monitor and maintain.