Splitting the database would require the separate server which means running the scripts on one and the database on the other. I'm not sure of anything else that can be done on that level to improve performance.
How many posts and registered users do you have? When you run top when things start to slow down what are you seeing. Memory swapping or an overutilized cpu? I'm just assuming you are running linux so this should give your current cpu load as well. You will also be able to see what is causing the slowdown, either the mysql server or individual scripts.
A couple of other configuration options that do help improve speed is have the default to show collapsed threads on the postlist screen, about 20-25 per page and flat mode.
