We have a new server arriving shortly so i will be looking to migrate the server 2003 over to 2003 R2 to see if it helps any. I will also be looking at the total amount of RAm that we have in there but we have ordered 2 so it may be able to get a secondary DC on the go to ease the strain a little.
A few more things that popped into my head...
If you're putting all those roles onto one server, and there's no reason why you can't, I'd pop some more ram in it just to give it some headroom. How much free memory is showing in task manager right now?
What's the network link to the server? 100mb, 1gbit? If your server has dual NICs you might be able to team them and almost double the traffic you can handle during peak times.